| Document and Entity Information | Apr. 22, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 22, 2021 |
| Entity Registrant Name | CAPSTAR FINANCIAL HOLDINGS, INC. |
| Entity Central Index Key | 0001676479 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity File Number | 001-37886 |
| Entity Incorporation, State or Country Code | TN |
| Entity Tax Identification Number | 81-1527911 |
| Entity Address, Address Line One | 1201 Demonbreun Street, Suite 700 |
| Entity Address, City or Town | Nashville |
| Entity Address, State or Province | TN |
| Entity Address, Postal Zip Code | 37203 |
| City Area Code | (615) |
| Local Phone Number | 732-6400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $1.00 par value per share |
| Trading Symbol | CSTR |
| Security Exchange Name | NASDAQ |
