| Document and Entity Information | Nov. 18, 2021 |
| Cover [Abstract] | |
| Entity Address, State or Province | TX |
| Amendment Flag | false |
| Entity Central Index Key | 0001499453 |
| Document Type | 8-K |
| Document Period End Date | Nov. 18, 2021 |
| Entity Registrant Name | Spirit of Texas Bancshares, Inc. |
| Entity Incorporation State Country Code | TX |
| Entity File Number | 001-38484 |
| Entity Tax Identification Number | 90-0499552 |
| Entity Address, Address Line One | 1836 Spirit of Texas Way |
| Entity Address, City or Town | Conroe |
| Entity Address, Postal Zip Code | 77301 |
| City Area Code | (936) |
| Local Phone Number | 521-1836 |
| Written Communications | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, no par value |
| Trading Symbol | STXB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
