Document and Entity Information - shares
6 Months Ended
Jun. 30, 2018
Jul. 26, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name CBTX, Inc.  
Entity Central Index Key 0001473844  
Document Type 10-Q  
Document Period End Date Jun. 30, 2018  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   25,962,584
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2