| Document and Entity Information | Sep. 22, 2022 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 22, 2022 |
| 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 |
| Entity Address, Adress Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Trading Symbol | CBTX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001473844 |
| Amendment Flag | false |
