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Covered Assets and FDIC Loss-sharing Asset (FDIC Loss-sharing Asset) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2016 | Mar. 31, 2015
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at beginning of period | $ 6,568 | $ 15,174
Cash (received from) paid to the FDIC, net | (353) | (659)
FDIC reimbursable losses, net | 136 | (21)
Amortization, net | (1,332) | (2,294)
Loan impairment | 147 | 1,532
Sale of other real estate | (144) | (420)
Write-downs of other real estate | 18 | 1,071
Other | 80 | (261)
Balance at end of period | 5,954 | $ 14,644
Amount of Estimated Clawback Liability | 5,418 |
Non-Single Family Covered Assets | 101,163 |
Single Family Covered Assets | 38,106 |
Columbia River Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 537 |
Amount of Estimated Clawback Liability | 3,420 |
Non-Single Family Covered Assets | 66,682 |
Single Family Covered Assets | 7,764 |
American Marine Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 2,404 |
Amount of Estimated Clawback Liability | 1,264 |
Non-Single Family Covered Assets | 11,321 |
Single Family Covered Assets | 22,561 |
Summit Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 1,799 |
Amount of Estimated Clawback Liability | 0 |
Non-Single Family Covered Assets | 8,381 |
Single Family Covered Assets | 6,046 |
First Heritage Bank [Member] |  |
Covered Assets And FDIC Loss Sharing Asset [Line Items] |  |
Balance at end of period | 1,214 |
Amount of Estimated Clawback Liability | 734 |
Non-Single Family Covered Assets | 14,779 |
Single Family Covered Assets | $ 1,735 |

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- Definition Amount of estimated clawback liability + References No definition available. + Details
Name: | colb_AmountOfEstimatedClawbackLiability
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
Period Type: | duration

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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: |