Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Feb. 24, 2019
Jun. 30, 2018
Document and Entity Information [Abstract]      
Entity Registrant Name CBTX, Inc.    
Entity Central Index Key 0001473844    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Accelerated Filer    
Entity Public Float     $ 586.2
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Common Stock, Shares Outstanding   26,002,653  
Entity Shell Company false    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY