| Document and Entity Information - USD ($) $ in Millions | 12 Months Ended | ||
| Dec. 31, 2021 | Mar. 07, 2022 | Jun. 30, 2021 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Document Period End Date | Dec. 31, 2021 | ||
| Document Fiscal Year Focus | 2021 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Registrant Name | Gritstone bio, Inc. | ||
| Entity Central Index Key | 0001656634 | ||
| Current Fiscal Year End Date | --12-31 | ||
| Title of 12(b) Security | Common Stock, $0.0001 par value per share | ||
| Trading Symbol | GRTS | ||
| Security Exchange Name | NASDAQ | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Voluntary Filers | No | ||
| ICFR Auditor Attestation Flag | false | ||
| Entity Filer Category | Accelerated Filer | ||
| Entity Common Stock, Shares Outstanding | 72,708,342 | ||
| Entity Small Business | true | ||
| Entity Emerging Growth Company | true | ||
| Entity Ex Transition Period | true | ||
| Entity Shell Company | false | ||
| Entity Public Float | $ 419 | ||
| Entity File Number | 001-38663 | ||
| Entity Incorporation, State or Country Code | DE | ||
| Entity Tax Identification Number | 47-4859534 | ||
| Entity Address, Address Line One | 5959 Horton Street | ||
| Entity Address, Address Line Two | Suite 300 | ||
| Entity Address, City or Town | Emeryville | ||
| Entity Address, State or Province | CA | ||
| Entity Address, Postal Zip Code | 94608 | ||
| City Area Code | 510 | ||
| Local Phone Number | 871-6100 | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Documents Incorporated by Reference | Portions of the Registrant’s Definitive Proxy Statement relating to the Annual Meeting of Shareholders, scheduled to be held on June 17, 2022, are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission on April 28, 2022. | ||
| Amendment Description | Gritstone bio, Inc. (the “Company”, “we”, “our” and “us”) is filing this Amendment No. 1 (the “Amendment No. 1”) solely to amend our Annual Report on Form 10-K for the fiscal year ended December 31, 2021 (the “Original Filing”), originally filed with the Securities and Exchange Commission (the “SEC”) on March 10, 2022 (the “Original Filing Date”). We are filing this Amendment No. 1 to amend clerical errors made in Part I of the Original Filing. The clerical errors were made in the third sentence and the title of Figure 8 included under the heading “Item 1. Business—New Developments—GRANITE updates”.Part IV of the Original Filing is being amended solely to add new certifications in accordance with Rule 13a-14(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). Because no financial statements have been included in this Amendment No. 1 and this Amendment No. 1 does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4 and 5 of the certifications have been omitted. We are not including the certifications under Section 906 of the Sarbanes-Oxley Act of 2002 as no financial statements are being filed with this Amendment No. 1.Except as set forth herein, this Amendment No. 1 does not change the previously reported financial statements or any of the other disclosures contained in Part I, Part II, Part III or Part IV of the Original Filing. Unless expressly noted otherwise, the disclosures in this Amendment No. 1 continue to speak as of the Original Filing Date and do not reflect events occurring after the Original Filing Date. Accordingly, this Amendment No. 1 should be read in conjunction with the Original Filing, our subsequent filings with the SEC and any amendments thereto. | ||
| Auditor Name | Ernst & Young LLP | ||
| Auditor Location | Redwood City, California | ||
| Auditor Firm ID | 42 |
