v3.10.0.1
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2018
Aug. 03, 2018
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2  
Trading Symbol STXB  
Entity Registrant Name Spirit of Texas Bancshares, Inc.  
Entity Central Index Key 0001499453  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   9,787,449