Loans Loans (Analysis of Troubled Debt Restructurings) (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) Modifications	Dec. 31, 2019 USD ($) Modifications	Dec. 31, 2018 USD ($) Modifications
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	19	24	34
Pre-Modification Outstanding Recorded Investment	$ 7,566	$ 7,348	$ 22,047
Post-Modification Outstanding Recorded Investment	$ 7,566	$ 7,348	$ 22,047
Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	0	0	1
Pre-Modification Outstanding Recorded Investment	$ 0	$ 0	$ 891
Post-Modification Outstanding Recorded Investment	$ 0	$ 0	$ 891
Commercial Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	11	10	10
Pre-Modification Outstanding Recorded Investment	$ 3,257	$ 6,560	$ 18,352
Post-Modification Outstanding Recorded Investment	$ 3,257	$ 6,560	$ 18,352
Agriculture
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	2	0	2
Pre-Modification Outstanding Recorded Investment	$ 3,495	$ 0	$ 27
Post-Modification Outstanding Recorded Investment	$ 3,495	$ 0	$ 27
Real estate: One-to-four family residential
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	6	13	21
Pre-Modification Outstanding Recorded Investment	$ 814	$ 787	$ 2,777
Post-Modification Outstanding Recorded Investment	$ 814	$ 787	$ 2,777
Consumer Loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
TDR Modifications, Number of Contracts \| Modifications	0	1	0
Pre-Modification Outstanding Recorded Investment	$ 0	$ 1	$ 0
Post-Modification Outstanding Recorded Investment	$ 0	$ 1	$ 0

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_CommercialBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_AgricultureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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