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Securities (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
issuance
Dec. 31, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]      
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds $ 63,292,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds $ 166,881,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds $ 95,165,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Number Of Issuances Of Securities Exceeding Shareholders Equity Threshold 0colb_NumberOfIssuancesOfSecuritiesExceedingShareholdersEquityThreshold    
U.S. Government and Government-Sponsored Enterprise Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Number Of Securities In Unrealized Loss Position 34colb_NumberOfSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
   
Number Of Securities In Continuous Loss Position For Twelve Months or More 17colb_NumberOfSecuritiesInContinuousLossPositionForTwelveMonthsOrMore
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
   
US Treasury Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Number Of Securities In Continuous Loss Position For Twelve Months or More 2colb_NumberOfSecuritiesInContinuousLossPositionForTwelveMonthsOrMore
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
   
Other Securities [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Issuances Of Securities Exceeding Shareholders Equity Threshold, Percent 10.00%colb_IssuancesOfSecuritiesExceedingShareholdersEquityThresholdPercent
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= colb_OtherSecuritiesMember
   
Number Of Securities In Unrealized Loss Position 2colb_NumberOfSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= colb_OtherSecuritiesMember
   
Number Of Securities In Continuous Loss Position For Twelve Months or More 1colb_NumberOfSecuritiesInContinuousLossPositionForTwelveMonthsOrMore
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= colb_OtherSecuritiesMember
   
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Number Of Securities In Unrealized Loss Position 125colb_NumberOfSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
   
Number Of Securities In Continuous Loss Position For Twelve Months or More 43colb_NumberOfSecuritiesInContinuousLossPositionForTwelveMonthsOrMore
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
   
Municipal Bonds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Number Of Securities In Unrealized Loss Position 95colb_NumberOfSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
   
Number Of Securities In Continuous Loss Position For Twelve Months or More 44colb_NumberOfSecuritiesInContinuousLossPositionForTwelveMonthsOrMore
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
   
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities     $ 3,000,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember