Quarterly report pursuant to Section 13 or 15(d)

Document and Entity Information

Document and Entity Information - shares
3 Months Ended
Mar. 31, 2021
Apr. 26, 2021
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Mar. 31, 2021  
Document Fiscal Year Focus 2021  
Document Fiscal Period Focus Q1  
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 false  
Entity Emerging Growth Company true  
Entity Ex Transition Period false  
Entity Common Stock, Shares Outstanding   17,145,575
Entity Shell Company false  
Title of 12(b) Security Common Stock, no par value  
Security Exchange Name NASDAQ  
Entity File Number 001-38484  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
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 Quarterly Report true  
Document Transition Report false