| Cover Page | Nov. 10, 2021 |
| Cover [Abstract] | |
| Entity Central Index Key | 0001626199 |
| Amendment Flag | false |
| Document Type | 8-K |
| Document Period End Date | Nov. 10, 2021 |
| Entity Registrant Name | ALPINE IMMUNE SCIENCES, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-37449 |
| Entity Tax Identification Number | 20-8969493 |
| Entity Address, Address Line One | 188 East Blaine Street |
| Entity Address, Address Line Two | Suite 200 |
| Entity Address, City or Town | Seattle |
| Entity Address, State or Province | WA |
| Entity Address, Postal Zip Code | 98102 |
| City Area Code | 206 |
| Local Phone Number | 788-4545 |
| 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 per share |
| Trading Symbol | ALPN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
