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Securities (Carrying Value of Securities Pledged as Collateral) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]  
Carrying amount of securities pledged as collateral $ 517,101us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Washington and Oregon State Public Deposits [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Carrying amount of securities pledged as collateral 323,771us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ colb_SecuritiesPledgedAsCollateralByCounterpartyAxis
= colb_WashingtonAndOregonStatePublicDepositsMember
Federal Reserve Bank Advances [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Carrying amount of securities pledged as collateral 36,945us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ colb_SecuritiesPledgedAsCollateralByCounterpartyAxis
= us-gaap_FederalReserveBankAdvancesMember
Other Securities [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Carrying amount of securities pledged as collateral $ 156,385us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
/ colb_SecuritiesPledgedAsCollateralByCounterpartyAxis
= colb_OtherSecuritiesMember