| Document and Entity Information | Mar. 25, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001560241 |
| Document Type | 8-K |
| Document Period End Date | Mar. 25, 2021 |
| Entity Registrant Name | G1 THERAPEUTICS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38096 |
| Entity Tax Identification Number | 26-3648180 |
| Entity Address, Address Line One | 700 Park Offices |
| Entity Address, Address Line Two | Drive |
| Entity Address, Address Line Three | Suite 200 |
| Entity Address, City or Town | Research Triangle Park |
| Entity Address, State or Province | NC |
| Entity Address, Postal Zip Code | 27709 |
| City Area Code | (919) |
| Local Phone Number | 213-9835 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.0001 par value |
| Trading Symbol | GTHX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
