Number of Defendant Entities

No definition available.

Name:	colb_NumberOfDefendantEntities
Namespace Prefix:	colb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Standby Letters Of Credit Expire Thereafter

+ References

No definition available.

+ Details

Name:	colb_StandbyLettersOfCreditExpireThereafter
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Standby Letters Of Credit Expires Within One Year

+ References

No definition available.

+ Details

Name:	colb_StandbyLettersOfCreditExpiresWithinOneYear
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Standby Letters Of Credit, Financial Guarantees required to perform on

+ References

No definition available.

+ Details

Name:	colb_StandbyLettersOfCreditFinancialGuaranteesrequiredtoperformon
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478785/954-310-50-2

+ Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-21

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-20

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-10

+ Details

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

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-9

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_LitigationCaseAxis=colb_ICapEntitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_LitigationCaseAxis=colb_ProfessionalFinancialInvestorsInc.AndProfessionalInvestorsSecurityFundInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_ProductConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_SalesRevenueNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=colb_RealEstateLoansasPartofLoanportfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=colb_MultifamilyLoansAsPartOfLoanPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=colb_CommercialOfficeLoansAsPartOfLoanPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies and Related-Party Transactions (Narrative) (Details) $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2025 USD ($)	Dec. 31, 2024	Mar. 31, 2024 USD ($) entity	Jun. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Financial guarantees in connection with standby letters of credit	$ 0		$ 0
Standby letters of credit expiring within one year	209,300
Standby letters of credit expiring thereafter	33,400
Loss Contingency Accrual	56,600
Concentration Risk [Line Items]
Loss Contingency Accrual	56,600
iCap Entities [Member]
Concentration Risk [Line Items]
Number of Defendant Entities \| entity			34
Loss Contingency, Damages Sought, Value	290,000
Professional Financial Investors, Inc. and Professional Investors Security Fund, Inc. [Member]
Commitments and Contingencies Disclosure [Abstract]
Loss Contingency Accrual	55,000
Concentration Risk [Line Items]
Loss Contingency Accrual	55,000
Minimum \| Professional Financial Investors, Inc. and Professional Investors Security Fund, Inc. [Member]
Concentration Risk [Line Items]
Loss Contingency, Damages Sought, Value	297,400
Maximum \| Professional Financial Investors, Inc. and Professional Investors Security Fund, Inc. [Member]
Concentration Risk [Line Items]
Loss Contingency, Damages Sought, Value	$ 368,100
Product Concentration Risk \| Revenue Benchmark \| Real Estate Loans As Part Of Loan Portfolio
Concentration Risk [Line Items]
Concentration risk (as a percent)	75.00%			75.00%
Product Concentration Risk \| Revenue Benchmark \| Multifamily Loans as Part of Loan portfolio [Member]
Concentration Risk [Line Items]
Concentration risk (as a percent)	20.00%	19.00%		20.00%
Product Concentration Risk \| Revenue Benchmark \| Commercial Office Loans as Part of Loan portfolio [Member]
Concentration Risk [Line Items]
Concentration risk (as a percent)	8.00%			8.00%