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Covered Assets and FDIC Loss-sharing Asset (FDIC Loss-sharing Asset) (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] |  |  |  |  |  |
Balance at beginning of period | $ 0 |  | $ 4,266 | $ 3,535 | $ 6,568 |
Cash paid to (received from) the FDIC, net | 0 |  | 20 | 184 | (23) |
FDIC reimbursable losses, net | 0 |  | (590) | (149) | (756) |
FDIC indemnification asset settlement | 0 | $ (3,123) | 0 |  | 0 |
Payment from (to) FDIC to terminate loss-sharing agreements |  |  |  | (4,666) | 0 |
Amortization, net | 0 |  | (315) | (414) | (2,530) |
Loan impairment | 0 |  | 266 | 40 | 393 |
Sale of other real estate | 0 |  | (49) | 18 | 71 |
Valuation adjustments of other real estate owned | 0 |  | 0 | 0 | (22) |
Other | 0 |  | 6 | 91 | 109 |
Balance at end of period | $ 0 | $ 0 | $ 3,592 | $ 0 | $ 3,592 |
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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Data Type: | xbrli:stringItemType
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- Definition FDIC Loss-sharing Asset, Cash Payments Paid to (Received from) the FDIC, net + References No definition available. + Details
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- Definition FDIC Loss-sharing Asset, Impairment (Recapture) of Loans + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition FDIC Loss-sharing Asset, Write-downs (Write-ups) of Other Real Estate + References No definition available. + Details
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Period Type: | duration

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- Definition FDIC indemnification asset settlement + References No definition available. + Details
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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition FDIC Loss-sharing Asset, Amortization, Net + References No definition available. + Details
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- Definition FDIC Loss-sharing Asset, Disposals + References No definition available. + Details
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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition FDIC Loss-sharing Asset, Net Reimbursable Losses + References No definition available. + Details
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- Definition FDIC Loss-sharing Asset, Other + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
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- Definition Single Family Covered Assets + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount of receivable assets originating from the indemnification agreement with the Federal Deposit Insurance Corporation (FDIC), in which the FDIC is to partially cover losses. + References No definition available. + Details
Name: | us-gaap_FDICIndemnificationAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant