| Document and Entity Information | Apr. 26, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Amendment Flag | true |
| Document Period End Date | Apr. 26, 2022 |
| Entity Registrant Name | NGM Biopharmaceuticals, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38853 |
| Entity Tax Identification Number | 26-1679911 |
| Entity Address, Address Line One | 333 Oyster Point Boulevard |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 243-5555 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | NGM |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Description | On March 10, 2022, NGM Biopharmaceuticals, Inc. (the “Company”) filed a Current Report on Form 8-K (the “Original Form 8-K”) disclosing that, on March 10, 2022, the Company received notice from Fidelity Investments Institutional Services Company (“Fidelity”) advising of an upcoming blackout period (the “Blackout Period”) with respect to the NGM Biopharmaceuticals Matching Plan and the NGM Biopharmaceuticals 401(k) Plan (respectively, the “Matching Plan” and the “401(k) Plan”), each of which include funds that are invested in the Company’s common stock, due to the merger of the Matching Plan into the 401(k) Plan. The Company also disclosed in the Original Form 8-K that the Blackout Period was expected to end on May 13, 2022.This Amendment No. 1 to the Original Form 8-K (“Amendment”) is being filed solely to disclose that, on April 26, 2022, the Company received notice from Fidelity that the Blackout Period ended earlier than expected.Other than providing the update in Item 5.04 below, no updates have been made to the Original Form 8-K. Accordingly, this Amendment should be read in conjunction with the Original Form 8-K. |
| Entity Central Index Key | 0001426332 |
