LOANS - By portfolio segment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loans
Total loans	$ 2,936,670	$ 2,646,673
Percentage of loan	100.00%	100.00%
Less allowance for credit losses for loans	$ (40,637)	$ (25,280)	$ (23,693)	$ (24,778)
Less deferred loan fees and unearned discounts	(9,880)	(6,125)
Less loans held for sale	$ (2,673)	$ (1,463)
Loans And Leases Receivable Allowance, Percent	100.00%	100.00%
Loans, net	$ 2,883,480	$ 2,613,805
Accrued interest receivable	12,100	7,500
Commercial and industrial
Loans
Total loans	$ 742,957	$ 527,607
Percentage of loan	25.30%	19.90%
Less allowance for credit losses for loans	$ (13,035)	$ (7,671)	(7,719)	(7,257)
Loans And Leases Receivable Allowance, Percent	32.10%	30.30%
Commercial real estate
Loans
Total loans	$ 1,041,998	$ 900,746
Percentage of loan	35.50%	34.00%
Less allowance for credit losses for loans	$ (13,798)	$ (7,975)	(6,730)	(10,375)
Loans And Leases Receivable Allowance, Percent	34.00%	31.60%
Construction and development
Loans
Total loans	$ 522,705	$ 527,812
Percentage of loan	17.80%	19.90%
Less allowance for credit losses for loans	$ (6,089)	$ (4,446)	(4,298)	(3,482)
Loans And Leases Receivable Allowance, Percent	15.00%	17.60%
1-4 family residential
Loans
Total loans	$ 239,872	$ 280,192
Percentage of loan	8.20%	10.60%
Less allowance for credit losses for loans	$ (2,578)	$ (2,257)	(2,281)	(1,326)
Loans And Leases Receivable Allowance, Percent	6.30%	8.90%
Multifamily residential
Loans
Total loans	$ 258,346	$ 277,209
Percentage of loan	8.80%	10.50%
Less allowance for credit losses for loans	$ (2,513)	$ (1,699)	(1,511)	(1,419)
Loans And Leases Receivable Allowance, Percent	6.20%	6.70%
Consumer
Loans
Total loans	$ 33,884	$ 36,782
Percentage of loan	1.10%	1.40%
Less allowance for credit losses for loans	$ (440)	$ (388)	(387)	(566)
Loans And Leases Receivable Allowance, Percent	1.10%	1.50%
Agriculture
Loans
Total loans	$ 8,670	$ 9,812
Percentage of loan	0.30%	0.40%
Less allowance for credit losses for loans	$ (137)	$ (74)	(62)	(68)
Loans And Leases Receivable Allowance, Percent	0.30%	0.30%
Other
Loans
Total loans	$ 88,238	$ 86,513
Percentage of loan	3.00%	3.30%
Less allowance for credit losses for loans	$ (2,047)	$ (770)	$ (705)	$ (285)
Other
Loans
Less allowance for credit losses for loans	$ (2,047)	$ (770)
Loans And Leases Receivable Allowance, Percent	5.00%	3.10%

Percentage of allowance to cover probable credit losses on loans and leases.

No definition available.

Name:	cbtx_LoansAndLeasesReceivableAllowancePercent
Namespace Prefix:	cbtx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents loans held for sale, deducted from the gross amount of loans to arrive at net loans.

+ References

No definition available.

+ Details

Name:	cbtx_LoansHeldForSale
Namespace Prefix:	cbtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of loan proportion to total loan.

+ References

No definition available.

+ Details

Name:	cbtx_PercentageOfLoanProportionToTotalLoan
Namespace Prefix:	cbtx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Unearned discounts (other than cash or quantity discounts and the like), finance charges, and interest included in the face amount of receivables, that are shown as a deduction from the related receivables. For example, 1) finance charges booked as a receivable when a loan is made and recognized as income at a later date; and 2) interest charges deducted from the face loan amount, resulting in a discounted amount actually advanced to the borrower (wherein the receivable includes the amount actually advanced to the borrower and the as yet unearned interest income).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4519-111522

+ Details

Name:	us-gaap_DeferredDiscountsFinanceChargesAndInterestIncludedInReceivables
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_InterestReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of financing receivable, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4531-111522

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(a),(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesAndLoansReceivableGrossCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

+ Details

Name:	us-gaap_NotesReceivableNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_ConstructionAndDevelopmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_MultiFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_AgricultureLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=cbtx_OtherLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: