| Document and Entity Information | Oct. 17, 2024 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001645666 |
| Current Fiscal Year End Date | --12-31 |
| Document Type | 8-K |
| Document Period End Date | Oct. 17, 2024 |
| Entity Registrant Name | Kezar Life Sciences, Inc. |
| 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 |
| Security 12b Title | Common Stock, $0.001 par value |
| Trading Symbol | KZR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
