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Allowance for Loan and Lease Losses and 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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Allowance For Loan And Lease Losses And Unfunded Loan Commitments And Letters Of Credit |  |  |  |
Beginning balance | $ 3,555 | $ 2,780 | $ 2,705 | $ 2,930
Allowance for Loan and Lease Losses, Net Changes In Unfunded Commitments and Letters Of Credit Allowance | (75) | 125 | 775 | (25)
Ending balance | $ 3,480 | $ 2,905 | $ 3,480 | $ 2,905

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- Definition Allowance For Loan And Lease Losses And Unfunded Loan Commitments And Letters Of Credit [Abstract] + References No definition available. + Details
Name: | colb_AllowanceForLoanAndLeaseLossesAndUnfundedLoanCommitmentsAndLettersOfCredit
Namespace Prefix: | colb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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