| Document and Entity Information | Aug. 27, 2020 |
| Cover [Abstract] | |
| Security Exchange Name | NASDAQ |
| Amendment Flag | false |
| Entity Central Index Key | 0001260990 |
| Document Type | 8-K |
| Document Period End Date | Aug. 27, 2020 |
| Entity Registrant Name | Oncternal Therapeutics, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 000-50549 |
| Entity Tax Identification Number | 62-1715807 |
| Entity Address, Address Line One | 12230 El Camino Real |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | San Diego |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92130 |
| City Area Code | (858) |
| Local Phone Number | 434-1113 |
| 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 | ONCT |
| Entity Emerging Growth Company | false |
