Annual report pursuant to Section 13 and 15(d)

Document and Entity Information

Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Feb. 28, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Trading Symbol STXB    
Entity Registrant Name Spirit of Texas Bancshares, Inc.    
Entity Central Index Key 0001499453    
Current Fiscal Year End Date --12-31    
Entity Voluntary Filers No    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
Entity Common Stock, Shares Outstanding   17,455,887  
Entity Public Float     $ 346.1
Entity Shell Company false    
Entity Current Reporting Status Yes    
Entity Well-known Seasoned Issuer No    
Entity Interactive Data Current Yes    
Title of 12(b) Security Common Stock, no par value per share    
Security Exchange Name NASDAQ    
Entity File Number 001-38484    
Entity Incorporation, State or Country Code TX    
Entity Tax Identification Number 90-0499552    
Entity Address, Address Line One 1836 Spirit of Texas Way    
Entity Address, City or Town Conroe    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 77301    
City Area Code 936    
Local Phone Number 521-1836    
Document Annual Report true    
Document Transition Report false    
ICFR Auditor Attestation Flag false    
Auditor Firm ID 243    
Auditor Name BDO USA, LLP    
Auditor Location Spokane, Washington