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Allowance for Credit Losses and Allowance for Unfunded Commitments and Letters of Credit (Narrative) (Details) | 3 Months Ended
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Sep. 30, 2021 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |
Minimum loan balance of nonaccrual, collateral dependent loans considered for ACL | $ 500,000
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease) | $ (6,400,000)

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- Definition Minimum loan balance of nonaccrual, collateral dependent loans considered for ACL + References No definition available. + Details
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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Definition Amount of increase (decrease) in allowance for credit loss on financing receivable. + 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) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration