| Document and Entity Information | Jul. 22, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | FIRST FINANCIAL BANKSHARES INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000036029 |
| Document Type | 8-K |
| Document Period End Date | Jul. 22, 2021 |
| Entity Incorporation State Country Code | TX |
| Entity File Number | 0-7674 |
| Entity Tax Identification Number | 75-0944023 |
| Entity Address, Address Line One | 400 Pine Street |
| Entity Address, City or Town | Abilene |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 79601 |
| City Area Code | (325) |
| Local Phone Number | 627-7155 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01 par value |
| Trading Symbol | FFIN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
