Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 21, 2019
Document and Entity Information [Abstract]    
Entity Registrant Name CBTX, INC.  
Entity Central Index Key 0001473844  
Document Type 10-Q  
Document Period End Date Sep. 30, 2019  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   26,016,468
Entity Shell Company false  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3