Document and Entity Information
6 Months Ended
Jun. 30, 2018
Document And Entity Information [Abstract]  
Document Type S-4/A
Amendment Flag false
Document Period End Date Jun. 30, 2018
Trading Symbol STXB
Entity Registrant Name Spirit of Texas Bancshares, Inc.
Entity Central Index Key 0001499453
Entity Filer Category Non-accelerated Filer
Entity Emerging Growth Company true
Entity Small Business false
Entity Extended Transition Period false