Document and Entity Information - shares
3 Months Ended
Mar. 31, 2019
Apr. 24, 2019
Document and Entity Information [Abstract]    
Entity Registrant Name CBTX, Inc.  
Entity Central Index Key 0001473844  
Document Type 10-Q  
Document Period End Date Mar. 31, 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   25,999,297
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1