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Outstanding and Unvested [Member] Flagship Pioneering, Inc. 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Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Amendment Flag Amendment Flag Sale of stock issue price per share (in dollars per share) Sale of Stock, Price Per Share Number of shares authorized (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized 2020 Employee Stock Purchase Plan Two Thousand Twenty Employee Stock Purchase Plan [Member] Leases Lessee, Operating Leases [Text Block] Termination Date Trading Arrangement Termination Date Maximum aggregate number of shares added to plan (in shares) Share Based Compensation Arrangement By Share Based Payment Award Maximum Future Increase Shares Maximum aggregate number of shares that may be added to the plan in future years. Net cash provided by financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Percentage of automatic annual increase in number of shares Share Based Compensation Arrangement By Share Based Payment Automatic Annual Increase, Percentage Amount of shares authorized to be automatically increased on the first day of the fiscal year, under a share based payment arrangement, as a percentage of common stock outstanding on the last day of the previous fiscal year. Measure: Measure [Axis] Weighted-average discount rate—operating leases Operating Lease, Weighted Average Discount Rate, Percent Long-Lived Tangible Asset [Domain] Long-Lived Tangible Asset [Domain] Weighted average common shares outstanding—basic (in shares) Weighted Average Number of Shares Outstanding, Basic Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table] Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table] Summary of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Related Party Transaction [Domain] Related Party Transaction [Domain] Segment Reporting [Abstract] Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Other income, net: Nonoperating Income (Expense) [Abstract] Entity Tax Identification Number Entity Tax Identification Number Proprietary programs Proprietary Programs [Member] Proprietary Programs Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common stock, shares issued (in shares) Common Stock, Shares, Issued Weighted-average remaining lease term—operating leases (in years) Operating Lease, Weighted Average Remaining Lease Term Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Loss Contingencies Loss Contingencies [Line Items] Insider Trading Arrangements [Line Items] Name of each exchange on which registered Security Exchange Name Total liabilities Liabilities Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Cash and Cash Equivalents [Axis] Cash and Cash Equivalents [Axis] Discount of shares rate, percent Share-Based Compensation Arrangement by Share-Based Payment Award, Discount from Market Price, Offering Date Purchases of property and equipment included in accounts payable and accrued expenses Capital Expenditures Incurred but Not yet Paid Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Operating expenses: Operating Expenses [Abstract] Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Issuance of common stock from direct offering, net of offering costs Stock Issued During Period, Value, New Issues Personnel expenses Personnel [Member] Personnel Statement Statement [Line Items] Facilities and IT related expenses, net of sublease income Facilities And Information Related Expenses, Net Sublease Income Facilities And Information Related Expenses, Net Sublease Income Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] FMR LLC FMR LLC [Member] FMR LLC Cash and cash equivalents Cash and cash equivalents Cash and Cash Equivalent Counterparty Name [Axis] Counterparty Name [Axis] Milestones payable upon event Milestones Payable Upon Event The amount the Company will pay for regulatory milestones on each licensed product upon the occurrence of specific events as outlined in the license agreement. Number of additional shares authorized (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Additional Shares Authorized Common Stock Common Stock [Member] Estimated Fair Value Marketable securities: Debt Securities, Available-for-Sale Entity Incorporation, State Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Corporate notes and bonds Corporate Debt Securities [Member] Accrued professional fees Accrued Professional Fees, Current Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Research and Development Arrangement, Contract to Perform for Others, Type [Domain] Collaboration Revenue [Member] Collaboration Revenue