| Cover | Sep. 24, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 24, 2021 |
| Entity File Number | 001-37990 |
| Entity Registrant Name | Leap Therapeutics, Inc. |
| Entity Central Index Key | 0001509745 |
| Entity Tax Identification Number | 27-4412575 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 47 Thorndike Street |
| Entity Address, Address Line Two | Suite B1-1 |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02141 |
| City Area Code | 617 |
| Local Phone Number | 714-0360 |
| 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 |
| Trading Symbol | LPTX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | false |
