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, 2018
Mar. 08, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
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 Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Entity Ex Transition Period false    
Entity Common Stock, Shares Outstanding   12,178,103  
Entity Public Float     $ 182.2
Entity Shell Company false    
Entity Current Reporting Status Yes    
Entity Well-known Seasoned Issuer No