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Allowance for Credit Losses and Allowance for Unfunded Commitments and Letters of Credit (Changes in the Allowance for Unfunded Commitments and Letters of Credit) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |  |  |  |  |
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Jun. 30, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018
Schedule of Allowance for Unfunded Commitments and Letters of Credit [Line Items] |  |  |  |  |  |  |  |  |
Allowance For Unfunded Loan Commitments And Letters Of Credit | $ 9,600 | $ 3,580 | $ 9,600 | $ 3,580 | $ 8,800 |  | $ 3,430 | $ 3,980 | $ 4,330
Allowance for Loan and Lease Losses, Net Changes In Unfunded Commitments and Letters Of Credit Allowance | $ 800 | $ (400) | $ 4,600 | $ (750) |  |  |  |  |
Accounting Standards Update 2016-13 [Member] |  |  |  |  |  |  |  |  |
Schedule of Allowance for Unfunded Commitments and Letters of Credit [Line Items] |  |  |  |  |  |  |  |  |
Change in allowance for unfunded commitments and letters of credit due to adoption of new ASU |  |  |  |  | $ 0 | $ 1,570 |  | $ 0 | $ 0

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- Definition Allowance for Loan and Lease Losses, Net Changes In Unfunded Commitments and Letters Of Credit Allowance + References No definition available. + Details
Name: | colb_AllowanceForLoanAndLeaseLossesNetChangesInUnfundedCommitmentsAndLettersOfCreditAllowance
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Allowance For Unfunded Loan Commitments And Letters Of Credit + References No definition available. + Details
Name: | colb_AllowanceForUnfundedLoanCommitmentsAndLettersOfCredit
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Change in allowance for unfunded commitments and letters of credit due to adoption of new ASU + References No definition available. + Details
Name: | colb_ChangeinallowanceforunfundedcommitmentsandlettersofcreditduetoadoptionofnewASU
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition [Line Items] for Schedule of Allowance for Unfunded Commitments and Letters of Credit [Table] + References No definition available. + Details
Name: | colb_ScheduleofAllowanceforUnfundedCommitmentsandLettersofCreditLineItems
Namespace Prefix: | colb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |