Quarterly report pursuant to Section 13 or 15(d)

Document and Entity Information

Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 02, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
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 Accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   15,794,282
Entity Shell Company false  
Entity File Number 001-38484  
Entity Current Reporting Status Yes  
Entity Tax Identification Number 900499552  
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