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Loans (Analysis of Nonaccrual Loans) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans $ 31,828us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus $ 31,352us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Unpaid Principal Balance Nonaccrual Loans 37,798colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus 44,774colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
Consumer Portfolio Segment [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 2,868us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 2,939us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Unpaid Principal Balance Nonaccrual Loans 3,799colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 3,930colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Secured Loans [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 17,240us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember		 16,552us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
Unpaid Principal Balance Nonaccrual Loans 20,573colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember		 21,453colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
Unsecured Loans [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 189us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember		 247us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
Unpaid Principal Balance Nonaccrual Loans 211colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember		 269colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
One-to-Four Family Residential [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 4,429us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember		 2,822us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
Unpaid Principal Balance Nonaccrual Loans 6,109colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember		 5,680colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
Commercial Land [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 1,616us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember		 821us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
Unpaid Principal Balance Nonaccrual Loans 1,560colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember		 1,113colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
Income Property Multifamily [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 887us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember		 3,200us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
Unpaid Principal Balance Nonaccrual Loans 887colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember		 5,521colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
Owner Occupied [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 1,995us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember		 3,826us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
Unpaid Principal Balance Nonaccrual Loans 2,036colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember		 5,837colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
Land And Acquisition [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 871us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember		 95us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
Unpaid Principal Balance Nonaccrual Loans 871colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember		 112colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
Residential Construction [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 1,263us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember		 370us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
Unpaid Principal Balance Nonaccrual Loans 1,263colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember		 370colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
Owner Occupied Construction [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment Nonaccrual Loans 470us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember		  
Unpaid Principal Balance Nonaccrual Loans $ 489colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember		 $ 489colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember