| Document and Entity Information | Mar. 11, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 11, 2021 |
| Entity Registrant Name | KEZAR LIFE SCIENCES, INC. |
| Entity Central Index Key | 0001645666 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38542 |
| Entity Tax Identification Number | 47-3366145 |
| Entity Address, Address Line One | 4000 Shoreline Court |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address City Or Town | South San Francisco |
| Entity Address State Or Province | CA |
| Entity Address Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 822-5600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value |
| Trading Symbol | KZR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
