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Allowance for Credit Losses and Unfunded Commitments and Letters of Credit (Changes in the Allowance for Unfunded Commitments and Letters of Credit) (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 01, 2020 | Dec. 31, 2017
Schedule of allowance for unfunded commitments and letters of credit [Line Items] |  |  |  |  |
Balance, Allowance For Unfunded Loan Commitments And Letters Of Credit | $ 8,300 | $ 3,430 | $ 4,330 |  | $ 3,130
Net changes in the allowance for unfunded commitments and letters of credit | $ 3,300 | (900) | 1,200 |  |
Accounting Standards Update 2016-13 |  |  |  |  |
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 | $ 0 | $ 1,570 |

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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 Schedule of allowance for unfunded commitments and letters of credit + 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: |