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Fair Value Accounting and Measurement (Aggregate FV and aggregate unpaid principal balance of LHFS under FV option (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Fair Value Disclosures [Abstract] |  |
FV, Loans held for sale, FV option, Mandatory Delivery method | $ 14,760 | $ 0
Fair Value, Loans held for sale, FV option, Aggregate Unpaid Principal Balance | 14,252 | 0
Fair Value, Option, Aggregate Differences, Loans and Long-term Receivables | $ 508 | $ 0

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- Definition Fair Value, FV option, Aggregate Unpaid Balance, Loans and Long-term Receivables + 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 Fair Value, Loans Held for Sale, Mandatory Delivery Method, FV option + References No definition available. + Details
Name: | colb_FairValueLoansHeldForSaleMandatoryDeliveryMethodFVOption
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_FairValueDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of loans and long-term receivables (other than securities categorized as trading, available-for-sale or held-to-maturity) that have contractual principal amounts and for which the fair value option has been elected. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612 + Details
Name: | us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant