| Document and Entity Information | Sep. 28, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001759425 |
| Document Type | 8-K |
| Document Period End Date | Sep. 28, 2021 |
| Entity Registrant Name | Mirum Pharmaceuticals, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-38981 |
| Entity Tax Identification Number | 83-1281555 |
| Entity Address, Address Line One | 950 Tower Lane |
| Entity Address, Address Line Two | Suite 1050 |
| Entity Address, City or Town | Foster City |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94404 |
| City Area Code | (650) |
| Local Phone Number | 667-4085 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, par value $0.0001 per share |
| Trading Symbol | MIRM |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
