| Cover | Nov. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Nov. 4, 2021 |
| 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 | true |
| Entity Ex Transition Period | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001426332 |
