Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 18, 2022	Jun. 30, 2021
Document and Entity Information [Abstract]
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2021
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 Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 496.3
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,608,462
Auditor Name	GRANT THORNTON LLP
Auditor Firm ID	248
Auditor Location	Houston, Texas
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Central Index Key	0001473844
Amendment Flag	false