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v3.20.1

Subsequent Events (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Jun. 30, 2020 | Mar. 31, 2020
Subsequent Event [Line Items] |  |
Visa Class B shares, remaining shares |  | 95,043
Subsequent Event [Member] |  |
Subsequent Event [Line Items] |  |
Visa Class B shares, shares sold | 17,360 |
Visa Class B shares, gain on sale | $ 3.0 |
Visa Class B shares, remaining shares | 77,683 |
Visa Class B shares, Fair Value of remaining shares | $ 13.4 |
Visa Class B shares, total gain including sale and write up | $ 16.4 |

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- Definition Visa Class B shares, Fair Value of remaining shares + References No definition available. + Details
Name: | colb_VisaClassBsharesFairValueofremainingshares
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Visa Class B shares, gain on sale + References No definition available. + Details
Name: | colb_VisaClassBsharesgainonsale
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Visa Class B shares, remaining shares + References No definition available. + Details
Name: | colb_VisaClassBsharesremainingshares
Namespace Prefix: | colb_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Visa Class B shares, shares sold + References No definition available. + Details
Name: | colb_VisaClassBsharessharessold
Namespace Prefix: | colb_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Visa Class B shares, total gain including sale and write up + References No definition available. + Details
Name: | colb_VisaClassBsharestotalgainincludingsaleandwriteup
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |