| Document and Entity Information | May 28, 2023 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001724979 |
| Document Type | 8-K |
| Document Period End Date | May 28, 2023 |
| Entity Registrant Name | Rain Oncology Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-40356 |
| Entity Tax Identification Number | 82-1130967 |
| Entity Address, Address Line One | 8000 Jarvis Avenue |
| Entity Address, Address Line Two | Suite 204 |
| Entity Address, City or Town | Newark |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94560 |
| City Area Code | (510) |
| Local Phone Number | 953-5559 |
| 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 | RAIN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
