Entity Central Index Key Cover [Abstract] Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Title of 12(b) Security Security12b Title Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock Shares Outstanding Entity Shell Company Entity Shell Company Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Ex Transition Period Entity Ex Transition Period Entity File Number Entity File Number Entity Incorporation, State or Country Code Entity Incorporation State Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address Address Line1 Entity Address, City or Town Entity Address City Or Town Entity Address, State or Province Entity Address State Or Province Entity Address, Postal Zip Code Entity Address Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Document Quarterly Report Document Quarterly Report Document Transition Report Document Transition Report Accrued liabilities and other current liabilities. Lease Incentive payable noncurrent. Statement Of Financial Position [Abstract] Assets Assets [Abstract] Current assets: Assets Current [Abstract] Cash and cash equivalents Cash And Cash Equivalents At Carrying Value Marketable securities Marketable Securities Unbilled accounts receivable Unbilled Receivables Current Prepaid expenses and other current assets Prepaid Expense And Other Assets Current Total current assets Assets Current Property and equipment, net Property Plant And Equipment Net Restricted cash Restricted Cash Noncurrent Other assets Other Assets Noncurrent Total assets Assets Liabilities and stockholders’ equity Liabilities And Stockholders Equity [Abstract] Current liabilities: Liabilities Current [Abstract] Accounts payable Accounts Payable Current Accrued expenses and other current liabilities Accrued Liabilities And Other Current Liabilities Deferred lease incentive, current portion Lease Incentive Payable Current Deferred revenue, current portion Contract With Customer Liability Current Total current liabilities Liabilities Current Deferred rent, excluding current portion Deferred Rent Credit Noncurrent Deferred lease incentive, excluding current portion Lease Incentive Payable Noncurrent Deferred revenue, excluding current portion Contract With Customer Liability Noncurrent Other liabilities, excluding current portion Other Liabilities Noncurrent Total liabilities Liabilities Commitments and contingencies (Note 12) Commitments And Contingencies Stockholders’ equity: Stockholders Equity [Abstract] Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued or outstanding Preferred Stock Value Common stock, $0.001 par value; 200,000,000 shares authorized; 27,452,872 and 23,335,514 shares issued as of June 30, 2020 and December 31, 2019, respectively; 27,223,796 and 22,654,444 shares outstanding as of June 30, 2020 and December 31, 2019, respectively Common Stock Value Treasury stock, at cost; no shares as of June 30, 2020 and December 31, 2019 Treasury Stock Value Additional paid-in capital Additional Paid In Capital Common Stock Accumulated other comprehensive income Accumulated Other Comprehensive Income Loss Net Of Tax Accumulated deficit Retained Earnings Accumulated Deficit Total stockholders’ equity Stockholders Equity Total liabilities and stockholders’ equity Liabilities And Stockholders Equity Preferred stock, par value Preferred Stock Par Or Stated Value Per Share Preferred stock, shares authorized Preferred Stock Shares Authorized Preferred stock, shares issued Preferred Stock Shares Issued Preferred stock, shares outstanding Preferred Stock Shares Outstanding Common stock, par value Common Stock Par Or Stated Value Per Share Common stock, shares authorized Common Stock Shares Authorized Common stock, shares issued Common Stock Shares Issued Common stock, shares outstanding Common Stock Shares Outstanding Treasury stock, shares Treasury Stock Shares Income Statement [Abstract] Collaboration revenue Revenue From Contract With Customer Including Assessed Tax Operating expenses: Operating Expenses [Abstract] Research and development Research And Development Expense General and administrative General And Administrative Expense Total operating expenses Operating Expenses Loss from operations Operating Income Loss Other income, net Interest Income Expense Nonoperating Net Net loss Net Income Loss Cumulative convertible preferred stock dividends Preferred Stock Dividends Income Statement Impact Net loss attributable to common stockholders Net Income Loss Available To Common Stockholders Basic Net loss per share attributable to common stockholders, basic and diluted Earnings Per Share Basic And Diluted Weighted average number of common shares used in net loss per share attributable to common stockholders, basic and diluted Weighted Average Number Of Share Outstanding Basic And Diluted Comprehensive loss: Comprehensive Income Net Of Tax [Abstract] Net loss Other comprehensive income: Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract] Unrealized gain on marketable securities Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax Total other comprehensive income Other Comprehensive Income Loss Net Of Tax Comprehensive loss Comprehensive Income Net Of Tax Temporary equity, stock issued during period, shares, new issues. Statement Of Stockholders Equity [Abstract] Statement [Table] Statement [Table] Accumulated Deficit Statement Equity Components [Axis] Equity Component Equity Component [Domain] Common Stock Common Stock [Member] Treasury Stock Treasury Stock Common [Member] Additional Paid-In Capital Additional Paid In Capital [Member] Accumulated Other Comprehensive Income Loss Accumulated Other Comprehensive Income [Member] Accumulated Deficit Retained Earnings [Member] Class of Stock Statement Class Of Stock [Axis] Class of Stock Class Of Stock [Domain] Series A convertible preferred stock. Series A Convertible Preferred Stock Series A Convertible Preferred Stock [Member] Series B convertible preferred stock. Series B Convertible Preferred Stock Series B Convertible Preferred Stock [Member] Sale of Stock Subsidiary Sale Of Stock [Axis] Sale of Stock Sale Of Stock Name Of Transaction [Domain] Private Placement Private Placement [Member] Statement [Line Items] Statement [Line Items] Convertible preferred stock, Beginning balance, Shares Temporary Equity Shares Outstanding Convertible preferred stock, Beginning balance Temporary Equity Carrying Amount Attributable To Parent Beginning balance Beginning balance, Shares Shares Outstanding Issuance of convertible preferred stock Temporary Equity Stock Issued During Period Value New Issues Issuance of convertible preferred stock, Shares Temporary Equity Stock Issued During Period Shares New Issues Issuance of common stock Stock Issued During Period Value New Issues Issuance of common stock, Shares Stock Issued During Period Shares New Issues Repurchase of unvested restricted stock awards Stock Repurchased During Period Shares Retirement of treasury shares Treasury Stock Shares Retired Stock-based compensation expense Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value Unrealized loss on marketable securities Convertible preferred stock, Ending balance, Shares Convertible preferred stock, Ending balance Ending balance Ending balance, Shares Increase (decrease) in deferred rent and deferred lease incentive. Public offering costs unpaid. Private placement offering costs unpaid at end of period. Statement Of Cash Flows [Abstract] IPO I P O [Member] Operating activities Net Cash Provided By Used In Operating Activities [Abstract] Net loss Profit Loss Adjustments to reconcile net loss to net cash used in operating activities: Adjustments To Reconcile Net Income Loss To Cash Provided By Used In Operating Activities [Abstract] Depreciation expense Depreciation Stock-based compensation expense Share Based Compensation In-process research and development expenses Research And Development In Process Net amortization of premiums and discounts on marketable securities Accretion Amortization Of Discounts And Premiums Investments Changes in operating assets and liabilities: Increase Decrease In Operating Capital [Abstract] Unbilled accounts receivable Increase Decrease In Accounts Receivable Prepaid expenses and other current assets Increase Decrease In Prepaid Deferred Expense And Other Assets Other assets Increase Decrease In Other Operating Assets Accounts payable Increase Decrease In Accounts Payable Accrued expenses and other liabilities Increase Decrease In Accrued Liabilities And Other Operating Liabilities Deferred revenue Increase Decrease In Deferred Revenue Deferred rent and deferred lease incentive Increase Decrease In Deferred Rent And Deferred Lease Incentive Net cash used in operating activities Net Cash Provided By Used In Operating Activities Investing activities Net Cash Provided By Used In Investing Activities [Abstract] Purchases of marketable securities Payments To Acquire Marketable Securities Maturities of marketable securities Proceeds From Sale And Maturity Of Marketable Securities Purchases of property and equipment Payments To Acquire Property Plant And Equipment Transaction costs associated with asset acquisition Payments To Acquire Intangible Assets Net cash used in investing activities Net Cash Provided By Used In Investing Activities Financing activities Net Cash Provided By Used In Financing Activities [Abstract] Issuance costs associated with issuance of Series B convertible preferred stock Payments Of Stock Issuance Costs Proceeds from issuance of common stock in connection with private placement, net of placement agent fees and offering costs Proceeds From Issuance Of Private Placement Principal payments on capital lease obligations Repayments Of Long Term Capital Lease Obligations Proceeds from issuance of common stock under benefit plans, net Proceeds From Issuance Of Common Stock Net cash provided by (used in) financing activities Net Cash Provided By Used In Financing Activities Net decrease in cash, cash equivalents and restricted cash Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect Cash, cash equivalents, and restricted cash, beginning of period Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Including Disposal Group And Discontinued Operations Cash, cash equivalents, and restricted cash, end of period Supplemental cash flow information Supplemental Cash Flow Information [Abstract] Cash paid for interest Interest Paid Net Non-cash investing and financing activities: Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract] Acquisition of in process research and development through issuance of stock Noncash Or Part Noncash Acquisition Intangible Assets Acquired1 Property and equipment purchases unpaid at end of period Capital Expenditures Incurred But Not Yet Paid Public offering costs unpaid at end of period Public Offering Costs Unpaid Private placement offering costs unpaid at end of period Private Placement Offering Costs Unpaid Organization Consolidation And Presentation Of Financial Statements [Abstract] Nature of the Business and Basis of Presentation Organization Consolidation And Presentation Of Financial Statements Disclosure [Text Block] Accounting Policies [Abstract] Summary of Significant Accounting Policies Significant Accounting Policies [Text Block] Fair Value Disclosures [Abstract] Fair Value Measurements Fair Value Disclosures [Text Block] Investments Debt And Equity Securities [Abstract] Marketable Securities Cash Cash Equivalents And Marketable Securities [Text Block] Property Plant And Equipment [Abstract] Property and Equipment, Net Property Plant And Equipment Disclosure [Text Block] Balance Sheet Related Disclosures [Abstract] Additional Balance Sheet Detail Supplemental Balance Sheet Disclosures [Text Block] Temporary Equity Disclosure [Abstract] Preferred Stock Preferred Stock [Text Block] Equity [Abstract] Common Stock Stockholders Equity Note Disclosure [Text Block] Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] Stock-based Compensation Expense Disclosure Of Compensation Related Costs Share Based Payments [Text Block] Acceleron Collaboration Agreement Collaborative Arrangement Disclosure [Text Block] The entire disclosure for asset acquisition. Goodwill And Intangible Assets Disclosure [Abstract] Asset Acquisition Asset Acquisition Disclosure [Text Block] Commitments And Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments And Contingencies Disclosure [Text Block] Compensation And Retirement Disclosure [Abstract] Defined Contribution Plan Pension And Other Postretirement Benefits Disclosure [Text Block] Earnings Per Share [Abstract] Net Loss per Share Earnings Per Share [Text Block] Related Party Transactions [Abstract] Related-Party Transactions Related Party Transactions Disclosure [Text Block] Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Principles of Consolidation Consolidation Policy [Text Block] Summary of Significant Accounting Policies Basis Of Accounting Policy Policy [Text Block] Use of Estimates Use Of Estimates Fair Value of Financial Instruments Fair Value Of Financial Instruments Policy Marketable Securities Marketable Securities Policy Off-balance sheet risk and concentrations of credit risk. Off-Balance Sheet Risk and Concentrations of Credit Risk Off Balance Sheet Risk And Concentrations Of Credit Risk Policy Policy [Text Block] Recently adopted accounting pronouncements. Recently Adopted Accounting Pronouncements Recently Adopted Accounting Pronouncements Policy [Text Block] Recent Accounting Pronouncements-To Be Adopted New Accounting Pronouncements Policy Policy [Text Block] Summary of Financial Assets Measured at Fair Value on Recurring Basis and Level of Fair Value Hierarchy Classification Fair Value Assets Measured On Recurring Basis [Text Block] Summary of Marketable Securities Marketable Securities [Text Block] Schedule of Property and Equipment, Net Property Plant And Equipment [Text Block] Represents schedule of prepaid expenses and other current assets. Schedule of Prepaid Expenses and Other Current Assets Schedule Of Prepaid Expenses And Other Current Assets Table [Text Block] Schedule of accrued expenses and other current liabilities. Schedule of Accrued Expenses and Other Current Liabilities Schedule Of Accrued Expenses And Other Current Liabilities Table [Text Block] Schedule of Potential Conversion of Preferred Stock and Future Issuance of Common Stock Schedule Of Stock By Class [Text Block] Plan Name Plan Name [Axis] Plan Name Plan Name [Domain] Two thousand nineteen and two thousand sixteen stock incentive plan. 2019 Plan and 2016 Plan Two Thousand Nineteen And Two Thousand Sixteen Stock Incentive Plan [Member] Grants outside of two thousand sixteen stock incentive plan and two thousand nineteen stock incentive plan. Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan Grants Outside Of Two Thousand Sixteen Stock Incentive Plan And Two Thousand Nineteen Stock Incentive Plan [Member] Summary of Restricted Stock Activity Schedule Of Share Based Compensation Restricted Stock Units Award Activity Table [Text Block] Summary of Stock Option Activity Schedule Of Share Based Compensation Stock Options Activity Table [Text Block] Weighted Average Assumptions Used to Calculate Fair Value of Stock Option Schedule Of Share Based Payment Award Stock Options Valuation Assumptions Table [Text Block] Summary of Stock-Based Compensation Expense Recognized Schedule Of Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Text Block] Schedule of Future Minimum Lease Payments Lessee Operating Lease Liability Maturity Table [Text Block] Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Text Block] Nature of business and basis of presentation. Nature of business and basis of presentation. Nature Of Business And Basis Of Presentation [Table] Nature Of Business And Basis Of Presentation [Table] Convertible Preferred Stock Convertible Preferred Stock [Member] Related Party Related Party Transactions By Related Party [Axis] Related Party Related Party [Domain] Investor Investor [Member] Collaborative Arrangement and Arrangement Other than Collaborative Type Of Arrangement [Axis] Collaborative Arrangement and Arrangement Other than Collaborative Arrangements And Nonarrangement Transactions [Member] MyoKardia Collaboration Agreement. MyoKardia Collaboration Agreement Myo Kardia Collaboration Agreement [Member] Subsequent Event Type Subsequent Event Type [Axis] Subsequent Event Type Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Nature Of Business And Basis Of Presentation [Line Items] Nature Of Business And Basis Of Presentation [Line Items] Share price Shares Issued Price Per Share Net proceeds of common stock Proceeds From Issuance Initial Public Offering Convertible preferred stock, share outstanding Number of preferred stock converted into common stock Convertible Preferred Stock Shares Issued Upon Conversion Reverse stock split Stockholders Equity Reverse Stock Split Stock split Stockholders Equity Note Stock Split Conversion Ratio1 Gross proceeds of common stock Offering costs Accumulated deficit Upfront payment received. Prepaid research funding received. Upfront payment received Upfront Payment Received Prepaid research funding received Prepaid Research Funding Received Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table] Measurement Frequency Fair Value By Measurement Frequency [Axis] Measurement Frequency Fair Value Measurement Frequency [Domain] Recurring Fair Value Measurements Recurring [Member] Level 2 Fair Value By Fair Value Hierarchy Level [Axis] Fair Value Hierarchy and NAV Fair Value Measurements Fair Value Hierarchy [Domain] Level 1 Fair Value Inputs Level1 [Member] Level 2 Fair Value Inputs Level2 [Member] Level 3 Fair Value Inputs Level3 [Member] Asset Class Fair Value By Asset Class [Axis] Asset Class Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation By Asset Class [Domain] Money Market Funds Money Market Funds [Member] US Treasury Securities U S Treasury Securities [Member] Corporate bonds. Corporate Bonds Corporate Bonds [Member] Commercial Paper Commercial Paper [Member] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] Cash equivalents Cash And Cash Equivalents Fair Value Disclosure Marketable securities Available For Sale Securities Debt Securities Total Assets Fair Value Disclosure Fair value transfers between levels. Fair value transfers between levels Fair Value Transfers Between Levels Cash and cash equivalents amortized cost. Cash and cash equivalents accumulated gross unrealized gains before tax. Cash and cash equivalents accumulated gross unrealized losses before tax. Cash equivalents and marketable securities amortized cost basis. Cash equivalents and marketable securities accumulated gross unrealized gains before tax. Cash equivalents and marketable securities accumulated gross unrealized gains before tax. Cash equivalents and marketable securities fair value disclosure. Debt Securities Available For Sale [Table] Debt Securities Available For Sale [Table] Investment Type Investment Type [Axis] Investments Investment Type Categorization [Member] Corporate Bonds Corporate Bond Securities [Member] Cash and Cash Equivalents Cash And Cash Equivalents [Axis] Cash and Cash Equivalents Restricted Cash And Cash Equivalents Cash And Cash Equivalents [Member] Schedule Of Available For Sale Securities [Line Items] Schedule Of Available For Sale Securities [Line Items] Cash equivalents, Amortized Cost Cash And Cash Equivalents Amortized Cost Cash equivalents, Gross unrealized Gains Cash And Cash Equivalents Accumulated Gross Unrealized Gains Before Tax Cash equivalents, Gross unrealized Losses Cash And Cash Equivalents Accumulated Gross Unrealized Losses Before Tax Cash equivalents, Fair Value Marketable securities, Amortized Cost Available For Sale Debt Securities Amortized Cost Basis Marketable securities, Gross Unrealized Gains Available For Sale Debt Securities Accumulated Gross Unrealized Gain Before Tax Marketable securities, Gross Unrealized Losses Available For Sale Debt Securities Accumulated Gross Unrealized Loss Before Tax Marketable securities, Fair Value Total cash equivalents and marketable securities, Amortized Cost Cash Equivalents And Marketable Securities Amortized Cost Basis Total cash equivalents and marketable securities, Gross Unrealized Gains Cash Equivalents And Marketable Securities Accumulated Gross Unrealized Gains Before Tax Total cash equivalents and marketable securities, Gross Unrealized Losses Cash Equivalents And Marketable Securities Accumulated Gross Unrealized Loss Before Tax Total cash equivalents and marketable securities, Fair Value Cash Equivalents And Marketable Securities Fair Value Disclosure Marketable securities Marketable Securities Current Sale of marketable securities Aggregate fair value of securities, unrealized loss position for less than 12 months Debt Securities Available For Sale Continuous Unrealized Loss Position Less Than12 Months Aggregate fair value of securities, unrealized loss position for more than 12 months Debt Securities Available For Sale Continuous Unrealized Loss Position12 Months Or Longer Securities with other-than-temporary impairment Impairment Of Investments Schedule Of Property Plant And Equipment [Table] Schedule Of Property Plant And Equipment [Table] Long-Lived Tangible Asset Property Plant And Equipment By Type [Axis] Long-Lived Tangible Asset Property Plant And Equipment Type [Domain] Lab equipment. Lab Equipment Lab Equipment [Member] Furniture and Fixtures Furniture And Fixtures [Member] Computer Equipment Computer Equipment [Member] Software Software Development [Member] Leasehold Improvements Leasehold Improvements [Member] Construction in Process Construction In Progress [Member] Property Plant And Equipment [Line Items] Property Plant And Equipment [Line Items] Total property and equipment Property Plant And Equipment Gross Less: accumulated depreciation Accumulated Depreciation Depletion And Amortization Property Plant And Equipment Property and equipment, net Prepaid expenses. Prepaid sign-on bonuses subject to vesting provisions. Prepaid expenses Prepaid Expenses Prepaid sign-on bonuses subject to vesting provisions Prepaid Sign On Bonuses Subject To Vesting Provisions Interest income receivable Interest Receivable Other Other Assets Current Total prepaid expenses and other current assets Accrued research and development expenses. Accrued restricted stock liability, current. External research and development Accrued Research And Development Expenses Payroll and benefits Employee Related Liabilities Current Professional services Accrued Professional Fees Current Capital lease obligation, current portion Capital Lease Obligations Current Restricted stock liability, current portion Accrued Restricted Stock Liability Current Other Other Liabilities Current Total accrued expenses and other current liabilities Temporary Equity By Class Of Stock [Table] Temporary Equity By Class Of Stock [Table] Transaction Type Transaction Type [Axis] Transaction Transaction [Domain] GSK agreement. GSK Agreement G S K Agreement [Member] Series B Preferred Stock Series B Preferred Stock [Member] Series A Preferred Stock Series A Preferred Stock [Member] Temporary Equity [Line Items] Temporary Equity [Line Items] Issuance of convertible preferred stock Gross proceeds threshold of automatic conversion of preferred stock to common stock. Gross proceeds threshold of automatic conversion of preferred stock eliminated Gross Proceeds Threshold Of Automatic Conversion Of Preferred Stock To Common Stock Temporary equity, dividends declared. Temporary equity, dividends declared Temporary Equity Dividends Declared Number of common stock voting rights. Common stock voting rights Common Stock Voting Rights Number of common stock voting rights Number Of Common Stock Voting Rights Dividends declared or paid Dividends Shares reserved for conversion of preferred stock and future issuance of common stock. Schedule Of Stock By Class [Table] Schedule Of Stock By Class [Table] Grants under two thousand nineteen stock incentive plan. 2019 Stock Incentive Plan Grants Under Two Thousand Nineteen Stock Incentive Plan [Member] Grants under two thousand nineteen employee stock purchase plan. 2019 Employee Stock Purchase Plan Grants Under Two Thousand Nineteen Employee Stock Purchase Plan [Member] Award Type Award Type [Axis] Award Type Share Based Compensation Arrangements By Share Based Payment Award Award Type And Plan Name [Domain] Outstanding Stock Options Employee Stock Option [Member] Class Of Stock [Line Items] Class Of Stock [Line Items] Shares reserved for future issuance Common Stock Capital Shares Reserved For Future Issuance Shares reserved for conversion of preferred stock and future issuance of common stock Shares Reserved For Conversion Of Preferred Stock And Future Issuance Of Common Stock Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table] Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table] Two thousand sixteen stock incentive plan. 2016 Stock Incentive Plan Two Thousand Sixteen Stock Incentive Plan [Member] Two thousand nineteen stock incentive plan. 2019 Stock Incentive Plan Two Thousand Nineteen Stock Incentive Plan [Member] Stock Option Statistical Measurement Range [Axis] Statistical Measurement Range [Member] Maximum Maximum [Member] Minimum [Member] Minimum [Member] 2019 Employee Stock Purchase Plan Employee Stock [Member] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Share Based Compensation Arrangement By Share Based Payment Award [Line Items] Increase in common stock capital shares reserved for future issuance. Increase in number of shares reserved for issuance, percent of common stock outstanding. Increase in number of shares reserved for issuance (shares) Increase In Common Stock Capital Shares Reserved For Future Issuance Increase in number of shares reserved for issuance, percent of common stock outstanding Increase In Number Of Shares Reserved For Issuance Percent Of Common Stock Outstanding Share based compensation arrangement by share based payment award equity instruments other than options repurchased in period. Restricted Stock Restricted Stock [Member] Number of Shares, Unvested, Beginning balance Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number Number of Shares, Granted Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Number of Shares, Vested Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Number of Shares, Repurchased Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Repurchased In Period Number of Shares, Unvested, Ending balance Share based compensation arrangement by share based payment award equity instruments other than options repurchased in period weighted average grant date fair value. Weighted Average Grant Date Fair Value, Unvested, Beginning balance Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Granted Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Vested Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Repurchased Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Repurchased In Period Weighted Average Grant Date Fair Value Weighted Average Grant Date Fair Value, Unvested, Ending balance Vesting period Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Period1 Contractual term Sharebased Compensation Arrangement By Sharebased Payment Award Options Outstanding Weighted Average Remaining Contractual Term2 Number of Shares, Outstanding, Beginning balance Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Number Number of Shares, Granted Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Gross Number of Shares, Exercised Stock Issued During Period Shares Stock Options Exercised Number of Shares, Cancelled Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures And Expirations In Period Number of Shares, Outstanding, Ending balance Number of Shares, Exercisable Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Number Weighted Average Exercise Price, Outstanding, Beginning balance Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Weighted Average Exercise Price Weighted Average Exercise Price, Granted Share Based Compensation Arrangements By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price Weighted Average Exercise Price, Exercised Share Based Compensation Arrangements By Share Based Payment Award Options Exercises In Period Weighted Average Exercise Price Weighted Average Exercise Price, Cancelled Share Based Compensation Arrangement By Share Based Payment Award Options Forfeitures And Expirations In Period Weighted Average Exercise Price Weighted Average Exercise Price, Outstanding, Ending balance Weighted Average Exercise Price, Exercisable Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Weighted Average Exercise Price Weighted Average Remaining Contractual Term (in years), Outstanding Weighted Average Remaining Contractual Term (in years), Exercisable Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Weighted Average Remaining Contractual Term1 Aggregate Intrinsic Value, Outstanding Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Intrinsic Value Aggregate Intrinsic Value, Exercisable Sharebased Compensation Arrangement By Sharebased Payment Award Options Exercisable Intrinsic Value1 Weighted average grant date fair value Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Grant Date Fair Value Total intrinsic value of stock options exercised Share Based Compensation Arrangement By Share Based Payment Award Options Exercises In Period Total Intrinsic Value Risk-free interest rate Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Risk Free Interest Rate Expected dividend yield Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate Expected term (years) Sharebased Compensation Arrangement By Sharebased Payment Award Fair Value Assumptions Expected Term1 Expected stock price volatility Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Volatility Rate Aggregate intrinsic value of restricted stock awards vested Sharebased Compensation Arrangement By Sharebased Payment Award Equity Instruments Other Than Options Aggregate Intrinsic Value Vested Income Statement Location Income Statement Location [Axis] Income Statement Location Income Statement Location [Domain] Research and Development Research And Development Expense [Member] General and Administrative General And Administrative Expense [Member] Unrecognized stock-based compensation expense Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Unrecognized stock-based compensation expense, weighted average period expect to recognized Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition1 Research, development and sales arrangement aggregate amount receivable upon achievement of milestone. Range of tiered royalties on net sales. Non-refundable upfront payments received. Schedule Of Collaborative Arrangements And Noncollaborative Arrangement Transactions [Table] Schedule Of Collaborative Arrangements And Noncollaborative Arrangement Transactions [Table] Acceleron Collaboration Agreement Collaborative Arrangement [Member] Contingent Consideration by Type Contingent Consideration By Type [Axis] Contingent Consideration Type Contingent Consideration Type [Domain] Research milestone. Research Milestones Research Milestone [Member] Clinical and regulatory milestones. Clinical and Regulatory Milestones Clinical And Regulatory Milestones [Member] Worldwide net sales milestones. Worldwide Net Sales Milestones Worldwide Net Sales Milestones [Member] Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items] Non refundable upfront payment received Non Refundable Upfront Payments Received Aggregate amount receivable on achievement of milestone Research Development And Sales Arrangement Aggregate Amount Receivable Upon Achievement Of Milestone Range of tiered royalties on net sales Range Of Tiered Royalties On Net Sales Change in variable consideration constrained. Change in variable consideration constrained Change In Variable Consideration Constrained Deferred revenue current and noncurrent. Unbilled accounts receivable related to reimbursable research and development cost. Reimbursement Payment Recieved. Revenues Revenue From Contract With Customer Excluding Assessed Tax Deferred revenue current and non-current Deferred Revenue Current And Noncurrent Unbilled accounts receivable related to reimbursable research and development cost Unbilled Accounts Receivable Related To Reimbursable Research And Development Cost Reimbursement Payment Recieved Reimbursement Payment Recieved Royalty Expense Royalty Expense Clinical and regulatory milestone payables. Sales milestone. Clinical and regulatory milestone payables upon phase two clinical trial. Schedule Of Research And Development Assets Acquired Other Than Through Business Combination [Table] Schedule Of Research And Development Assets Acquired Other Than Through Business Combination [Table] Research And Development Assets Acquired Other Than Through Business Combination [Line Items] Research And Development Assets Acquired Other Than Through Business Combination [Line Items] Shares issued for asset acquisition Stock Issued During Period Shares Purchase Of Assets Shares issued for asset acquisition, estimated fair value Stock Issued During Period Value Purchase Of Assets Shares issued, price per share Specified clinical and regulatory milestones Clinical And Regulatory Milestone Payables Specified clinical and regulatory milestones phase two clinical trial Clinical And Regulatory Milestone Payables Upon Phase Two Clinical Trial Specified sales milestone Sales Milestone Direct expenses related to the transaction Acquisition Costs Area of properties held under operating leases. Commitments and contingencies. Commitments and contingencies. Commitments And Contingencies [Table] Commitments And Contingencies [Table] Geographical Statement Geographical [Axis] Geographical Segment Geographical [Domain] Cambridge, Massachusetts MASSACHUSETTS Other Commitments Other Commitments [Axis] Other Commitments Other Commitments [Domain] Member stands for commitment for office and laboratory space lease of current corporate headquarters. Commitment for Office and Laboratory Space Lease of Current Corporate Headquarters Commitment For Office And Laboratory Space Lease Of Current Corporate Headquarters [Member] Loss Contingency Nature Loss Contingencies By Nature Of Contingency [Axis] Loss Contingency, Nature Loss Contingency Nature [Domain] Indemnification Agreement Indemnification Guarantee [Member] Member stands for legal proceedings. Legal Proceedings Legal Proceedings [Member] Commitments And Contingencies [Line Items] Commitments And Contingencies [Line Items] Square feet of office and laboratory space leased under operating lease Area Of Properties Held Under Operating Leases Total commitment Lessee Operating Lease Liability Payments Due Operating lease term Lessee Operating Lease Term Of Contract Letters of credit Letters Of Credit Outstanding Amount Rent expense Lease And Rental Expense 2020 Lessee Operating Lease Liability Payments Remainder Of Fiscal Year 2021 Lessee Operating Lease Liability Payments Due Year Two 2022 Lessee Operating Lease Liability Payments Due Year Three 2023 Lessee Operating Lease Liability Payments Due Year Four 2024 Lessee Operating Lease Liability Payments Due Year Five Thereafter Lessee Operating Lease Liability Payments Due After Year Five Total minimum lease payments Accrued liabilities for indemnification agreements Loss Contingency Accrual At Carrying Value Expenses incurred for legal proceedings Loss Contingency Loss In Period Defined Contribution Plan Name Defined contribution plan name Defined Contribution Plan Name Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table] Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table] Antidilutive Securities Antidilutive Securities Excluded From Computation Of Earnings Per Share By Antidilutive Securities [Axis] Antidilutive Securities, Name Antidilutive Securities Name [Domain] Unvested Restricted Stock Awards Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items] Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items] Antidilutive securities excluded from calculation of diluted net loss per share Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount Schedule Of Related Party Transactions By Related Party [Table] Schedule Of Related Party Transactions By Related Party [Table] Third Rock Ventures LLC. TRV Third Rock Ventures L L C [Member] Related Party Transaction Related Party Transaction [Axis] Related Party Transaction Related Party Transaction [Domain] Consulting services. Consulting Services Consulting Services [Member] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Fees paid in exchange for consulting services Related Party Transaction Expenses From Transactions With Related Party Due to related party Due To Related Parties Current Common stock shares issued in exchange for services Stock Issued During Period Shares Issued For Services Upfront payment. Payment for Prepaid Research Funding. Milestone Payment For Certain Identified Targets. Milestone Payment for certain Other Identified Targets. Subsequent Event [Table] Subsequent Event [Table] MyoKardia Collaboration and License Agreement [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Upfront payment Upfront Payment Payment for prepaid research funding Payment For Prepaid Research Funding Milestone payment for certain identified targets Milestone Payment For Certain Identified Targets Milestone payment for certain other identified targets Milestone Payment For Certain Other Identified Targets