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Covered Assets and FDIC Loss-sharing Asset (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Jun. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Mar. 31, 2017
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |  |  |  |  |
Payment from (to) FDIC to terminate loss-sharing agreements |  |  |  | $ 4,666 | $ 0 |
FDIC indemnification asset settlement | $ 0 | $ 3,123 | $ 0 |  | 0 |
FDIC clawback liability removal upon termination of loss-sharing agreements |  | $ 5,400 |  |  |  |
FDIC loss-sharing agreement early termination, pretax charge |  |  |  | $ 2,409 | $ 0 |
Non-Single Family Covered Assets |  |  |  |  |  | $ 74,000
Single Family Covered Assets |  |  |  |  |  | $ 26,400

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- Definition Covered Assets And FDIC Loss Sharing Asset [Line Items] + References No definition available. + Details
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- Definition FDIC clawback liability removal upon termination of loss-sharing agreements + References No definition available. + Details
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- Definition FDIC indemnification asset settlement + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition FDIC loss-sharing agreement early termination, pretax charge + References No definition available. + Details
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- Definition Non-Single Family Covered Assets + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Payment from (to) FDIC to terminate loss-sharing agreements + References No definition available. + Details
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- Definition Single Family Covered Assets + References No definition available. + Details
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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant