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Securities Sold Under Agreements to Repurchase (Details) $ in Millions | Dec. 31, 2018 USD ($)
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Available-for-sale Securities [Member] |
Assets Sold under Agreements to Repurchase [Line Items] |
Assets Sold under Agreements to Repurchase, Term, Carrying Amounts | $ 25.0
Assets Sold under Agreements to Repurchase, Term, Interest Rate | 1.88%
Repurchase Agreements, Sweep [Member] |
Assets Sold under Agreements to Repurchase [Line Items] |
Carrying amount of securities pledged as collateral | $ 75.6
Repurchase Agreements, Sweep [Member] | Available-for-sale Securities [Member] |
Assets Sold under Agreements to Repurchase [Line Items] |
Assets Sold under Agreements to Repurchase, Sweep, Carrying Amount | $ 61.1
Assets Sold under Agreements to Repurchase, Sweep, Interest rate | 1.99%

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- Definition Carrying amount of assets sold under agreements to repurchase related to sweep products when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets. + References No definition available. + Details
Name: | colb_AssetsSoldunderAgreementstoRepurchaseSweepCarryingAmount
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Interest rate on the amount to be repaid to the counterparty to the sweep product repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References No definition available. + Details
Name: | colb_AssetsSoldunderAgreementstoRepurchaseSweepInterestrate
Namespace Prefix: | colb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Carrying amount of assets sold under term agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets. + References No definition available. + Details
Name: | colb_AssetsSoldunderAgreementstoRepurchaseTermCarryingAmount
Namespace Prefix: | colb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Interest rate on the amount to be repaid to the counterparty to the Term repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets. + References No definition available. + Details
Name: | colb_AssetsSoldunderAgreementstoRepurchaseTermInterestrate
Namespace Prefix: | colb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_AssetsSoldUnderAgreementsToRepurchaseLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for repurchase agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_AssetsSoldUnderAgreementsToRepurchaseAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=colb_RepurchaseAgreementsSweepMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |