| Document and Entity Information | May 24, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | NGM BIOPHARMACEUTICALS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001426332 |
| Document Type | 8-K |
| Document Period End Date | May 24, 2021 |
| Entity Incorporation State 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 |
| Security 12b Title | 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 |
