Fair Value Measurement (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Contractually Specified Servicing Fee, Late Fee, and Ancillary Fee Earned in Exchange for Servicing Financial Asset	$ 5	$ 6	$ 17	$ 18
Unrealized gains (losses) arising during the period	(8)	(2)	0	4
Changes in unrealized gains and losses on junior subordinated debentures carried at fair value, net of taxes	(6)	(1)	0	3
Fair Value, Option, Quantitative Disclosures [Line Items]
Net increase (decrease) in fair value	$ 4	9	11	(3)
Residential Mortgage Banking Revenue
Fair Value, Option, Quantitative Disclosures [Line Items]
Net increase (decrease) in fair value		$ 1	$ 1	$ 1

Amount of contractually specified servicing fee, late fee, and ancillary fee recognized as income for servicing asset and servicing liability.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481229/860-50-50-2

+ Details

Name:	us-gaap_ContractuallySpecifiedServicingFeesLateFeesAndAncillaryFeesEarnedInExchangeForServicingFinancialAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

+ Details

Name:	us-gaap_FairValueOptionChangesInFairValueGainLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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/exampleRef
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-SubTopic 10
-Topic 825
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482881/825-10-55-10

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482881/825-10-55-12

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 28
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-30

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-32

+ Details

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

- Definition

Amount, after tax, before reclassification adjustment, of unrealized gain (loss) from increase (decrease) in instrument-specific credit risk of financial liability measured under fair value option.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 15
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480991/815-15-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (l)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482736/825-10-45-5

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (d)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-30

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodAfterTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, before tax and reclassification adjustment, of unrealized gain (loss) from increase (decrease) in instrument-specific credit risk of financial liability measured under fair value option.

+ References

+ Details

Name:	us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementLocationAxis=colb_FeesAndCommissionsMortgageBankingAndServicingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: