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Covered Assets and FDIC Loss-sharing Asset (FDIC Loss-sharing Asset) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2017 | Mar. 31, 2016
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at beginning of period | $ 3,535 | $ 6,568
Cash paid to the FDIC, net | 184 | 353
FDIC reimbursable losses, net | (206) | 136
Amortization, net | (315) | (1,332)
Loan impairment | 40 | 147
Sale of other real estate | 7 | 144
Valuation adjustments of other real estate owned | 0 | 18
Other | 6 | 80
Balance at end of period | 3,239 | $ 5,954
Amount of Estimated Clawback Liability | 5,380 |
Non-Single Family Covered Assets | 73,991 |
Single Family Covered Assets | 26,445 |
Columbia River Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 158 |
Amount of Estimated Clawback Liability | 3,362 |
Non-Single Family Covered Assets | 50,590 |
Single Family Covered Assets | 5,721 |
American Marine Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 1,676 |
Amount of Estimated Clawback Liability | 1,275 |
Non-Single Family Covered Assets | 8,885 |
Single Family Covered Assets | 15,319 |
Summit Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 1,313 |
Amount of Estimated Clawback Liability | 0 |
Non-Single Family Covered Assets | 4,788 |
Single Family Covered Assets | 4,182 |
First Heritage Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 92 |
Amount of Estimated Clawback Liability | 743 |
Non-Single Family Covered Assets | 9,728 |
Single Family Covered Assets | $ 1,223 |

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- Definition Amount of estimated clawback liability + References No definition available. + Details
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Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Covered Assets And FDIC Loss Sharing Asset [Line Items] + References No definition available. + Details
Name: | colb_CoveredAssetsAndFdicLossSharingAssetLineItems
Namespace Prefix: | colb_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition FDIC Loss-sharing Asset, Impairment (Recapture) of Loans + References No definition available. + Details
Name: | colb_FDICLosssharingAssetImpairmentRecaptureofLoans
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
Name: | colb_FdicLossSharingAssetAmortizationNet
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition FDIC Loss-sharing Asset, Cash Payments Received + References No definition available. + Details
Name: | colb_FdicLossSharingAssetCashPaymentsReceived
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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

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

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- Definition FDIC Loss-sharing Asset, Write-downs of Other Real Estate + References No definition available. + Details
Name: | colb_FdicLossSharingAssetWriteDownsOfOtherRealEstate
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition Single Family Covered Assets + References No definition available. + Details
Name: | colb_SingleFamilyCoveredAssets
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
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

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=colb_ColumbiaRiverBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=colb_AmericanMarineBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=colb_SummitBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=colb_FirstHeritageBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |