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Loans (Analysis of Nonaccrual Loans) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status $ 31,352us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus  
Unpaid Principal Balance Nonaccrual Loans 44,774colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus 52,434colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
Recorded Investment Nonaccrual Loans   34,015colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
Consumer Portfolio Segment [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 2,939us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		  
Unpaid Principal Balance Nonaccrual Loans 3,930colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember		 6,187colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Recorded Investment Nonaccrual Loans   3,991colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Secured Loans [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 16,552us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember		  
Unpaid Principal Balance Nonaccrual Loans 21,453colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember		 19,186colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
Recorded Investment Nonaccrual Loans   12,433colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
Unsecured Loans [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 247us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember		  
Unpaid Principal Balance Nonaccrual Loans 269colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember		 202colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
Recorded Investment Nonaccrual Loans   176colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
One-to-Four Family Residential [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 2,822us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember		  
Unpaid Principal Balance Nonaccrual Loans 5,680colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember		 4,678colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
Recorded Investment Nonaccrual Loans   2,667colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
Commercial Land [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 821us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember		  
Unpaid Principal Balance Nonaccrual Loans 1,113colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember		 783colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
Recorded Investment Nonaccrual Loans   442colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
Income Property Multifamily [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 3,200us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember		  
Unpaid Principal Balance Nonaccrual Loans 5,521colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember		 5,383colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
Recorded Investment Nonaccrual Loans   4,267colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
Owner Occupied [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 3,826us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember		  
Unpaid Principal Balance Nonaccrual Loans 5,837colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember		 7,486colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
Recorded Investment Nonaccrual Loans   6,334colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
Land And Acquisition [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 95us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember		  
Unpaid Principal Balance Nonaccrual Loans 112colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember		 6,601colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
Recorded Investment Nonaccrual Loans   3,246colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
Residential Construction [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 370us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember		  
Unpaid Principal Balance Nonaccrual Loans 370colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember		 1,928colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
Recorded Investment Nonaccrual Loans   459colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
Owner Occupied Construction [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Recorded Investment, Nonaccrual Status 480us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember		  
Unpaid Principal Balance Nonaccrual Loans 489colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember		 0colb_FinancingReceivableUnpaidPrincipalBalanceNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
Recorded Investment Nonaccrual Loans   $ 0colb_FinancingReceivableRecordedInvestmentGrossNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember