Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements (Detail) - Assets Measured at Fair Value on a Nonrecurring Basis [Member] - (Level 3) [Member] $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)
Mortgage Servicing Rights [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value	$ 2,246	$ 2,224
Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Mortgage Servicing Rights [Member] \| Constant Prepayment Rate [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	12	8
Mortgage Servicing Rights [Member] \| Constant Prepayment Rate [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	50	28
Mortgage Servicing Rights [Member] \| Constant Prepayment Rate [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	22	16
Mortgage Servicing Rights [Member] \| Discount Rate [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	12	12
Mortgage Servicing Rights [Member] \| Discount Rate [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	12	12
Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value	$ 1
Valuation Technique [Extensible List]	civb:AppraisalOfCollateralMember
Impaired Loans [Member] \| Measurement Input, Appraised Value [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	0
Impaired Loans [Member] \| Measurement Input, Appraised Value [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	30
Impaired Loans [Member] \| Measurement Input, Appraised Value [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	19
Impaired Loans [Member] \| Measurement Input Holding Period [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans, Term	23 months
Impaired Loans [Member] \| Measurement Input Holding Period [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Impaired loans, Term	23 months
Other Real Estate Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value	$ 31
Valuation Technique [Extensible List]	civb:AppraisalOfCollateralMember
Other Real Estate Owned [Member] \| Measurement Input, Appraised Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	10
Other Real Estate Owned [Member] \| Measurement Input, Appraised Value [Member] \| Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Unobservable Input	10

Servicing asset, term.

No definition available.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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- 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

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- Definition

Value of input used to measure servicing asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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- Definition

Indicates valuation technique for measuring servicing asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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