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Subsequent Events (Details) - Pacific Continental [Member] - Subsequent Event [Member] $ in Thousands | Jan. 09, 2017 USD ($)
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Subsequent Event [Line Items] |
Business Acquisition, Date of Acquisition Agreement | Jan. 09, 2017
Business Combination, merger consideration, share equivalent, in acquirer's shares | 64.30%
Business Combination, total merger consideration estimate | $ 644,100
Business Combination, termination fee | $ 18,750

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- Definition Business Combination, merger consideration, share equivalent, in acquirer's shares + References No definition available. + Details
Name: | colb_BusinessCombinationmergerconsiderationshareequivalentinacquirersshares
Namespace Prefix: | colb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Business Combination, termination fee + References No definition available. + Details
Name: | colb_BusinessCombinationterminationfee
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Business Combination, total merger consideration estimate + References No definition available. + Details
Name: | colb_BusinessCombinationtotalmergerconsiderationestimate
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Date when the business acquisition agreement was executed, in CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |