| Document and Entity Information | Sep. 19, 2019 |
| 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 | Sep. 19, 2019 |
| Entity Incorporation, State or 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 | true |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.01 par value |
| Trading Symbol | FFIN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
