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SECURITIES - Securities carrying amount (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) item | Dec. 31, 2016 USD ($) item | Dec. 31, 2015 USD ($)
SECURITIES |  |  |
Securities sold during the year | $ 6,100,000 | $ 302,000 | $ 0
Net gains | 27,000 | 28,000 |
Carrying value of securities pledged | $ 58,700,000 | $ 54,000,000 |
Securities held in a gross unrealized loss position | item | 106 | 121 |
Impairment loss on securities | $ 0 | $ 0 | $ 0

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- Definition The carrying value of securities pledged to secure public deposits and for other purposes required or permitted by law. + References No definition available. + Details
Name: | cbtx_CarryingValueOfSecuritiesPledged
Namespace Prefix: | cbtx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of carrying value of Securities sold. + References No definition available. + Details
Name: | cbtx_CarryingValueOfSecuritiesSold
Namespace Prefix: | cbtx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of securities held in a gross unrealized loss position. + References No definition available. + Details
Name: | cbtx_SecuritiesHeldInGrossUnrealizedLossPosition
Namespace Prefix: | cbtx_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
Name: | us-gaap_GainLossOnSaleOfSecuritiesNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.M) -URI http://asc.fasb.org/extlink&oid=65877573&loc=d3e87990-122713 + Details
Name: | us-gaap_MarketableSecuritiesRealizedGainLossOtherThanTemporaryImpairmentsAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration