Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 23, 2020
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2020
Entity File Number	001-38280
Entity Registrant Name	CBTX, INC.
Entity Incorporation, State or Country Code	TX
Entity Tax Identification Number	20-8339782
Entity Address, Address Line One	9 Greenway Plaza, Suite 110
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77046
City Area Code	713
Local Phone Number	210-7600
Title of 12(b) Security	Common stock, par value $0.01 per share
Trading Symbol	CBTX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,844,548
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001473844
Amendment Flag	false