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Loans (Analysis of the Aged Loan Portfolio) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans $ 31,828us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus $ 31,352us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 5,171,371us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
5,158,985us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 23,760us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
14,656us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 3,661us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
8,415us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 436us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,386us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 27,857us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
24,457us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 31,828us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
31,352us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 5,231,056us-gaap_LoansReceivableNet
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
5,214,794us-gaap_LoansReceivableNet
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Secured Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 17,240us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
16,552us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
Secured Loans [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 2,024,277us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
2,004,418us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 11,910us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
5,137us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 1,874us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
6,149us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 58us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,372us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 13,842us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
12,658us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 17,240us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
16,552us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 2,055,359us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
2,033,628us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_SecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Unsecured Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 189us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
247us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
Unsecured Loans [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 78,351us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
79,661us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 492us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
185us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 120us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 67us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 679us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
185us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 189us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
247us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 79,219us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
80,093us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_UnsecuredLoansMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
One-to-Four Family Residential [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 4,429us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
2,822us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
One-to-Four Family Residential [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 164,962us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
167,197us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 771us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,700us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 124us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
45us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 21us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 916us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,745us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 4,429us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
2,822us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 170,307us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
171,764us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OneToFourFamilyResidentialMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Commercial Land [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 1,616us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
821us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
Commercial Land [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 186,904us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
187,470us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 4,303us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,454us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 355us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
34us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 264us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 4,922us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,488us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 1,616us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
821us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 193,442us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
189,779us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_CommercialLandMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Income Property Multifamily [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 887us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
3,200us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
Income Property Multifamily [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 1,305,595us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,294,982us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 2,502us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
3,031us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 560us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
786us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 3,062us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
3,817us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 887us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
3,200us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 1,309,544us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,301,999us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Owner Occupied [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 1,995us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
3,826us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
Owner Occupied [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 841,551us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
839,689us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 2,206us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
937us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 435us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
289us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 2,641us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,226us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 1,995us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
3,826us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 846,187us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
844,741us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Land And Acquisition [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 871us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
95us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
Land And Acquisition [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 15,860us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
15,462us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 67us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
953us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 67us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
953us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 871us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
95us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 16,798us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
16,510us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_LandAndAcquisitionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Residential Construction [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 1,263us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
370us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
Residential Construction [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 104,378us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
97,821us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
326us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 4us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
4us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 4us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
330us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 1,263us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
370us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 105,645us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
98,521us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_ResidentialConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Income Property Multifamily Construction [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 62,958us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
73,783us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 0us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 0us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 62,958us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
73,783us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_IncomePropertyMultifamilyConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Owner Occupied Construction [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 470us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
 
Owner Occupied Construction [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 54,969us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
57,470us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
994us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
0us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 0us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
994us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 470us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
480us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net 55,439us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
58,944us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= colb_OwnerOccupiedConstructionMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Consumer Portfolio Segment [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Nonaccrual Loans 2,868us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
2,939us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
Consumer Portfolio Segment [Member] | Loans, Excluding Purchased Credit Impaired Loans [Member]    
Financing Receivable, Recorded Investment, Past Due [Line Items]    
Current Loans 331,566us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
341,032us-gaap_FinancingReceivableRecordedInvestmentCurrent
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
30 - 59 Days Past Due 1,509us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
933us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
60 - 89 Days Past Due 193us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
118us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due 22us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
10us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Total Past Due 1,724us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
1,061us-gaap_FinancingReceivableRecordedInvestmentPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Nonaccrual Loans 2,868us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
2,939us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
Loans Receivable, Net $ 336,158us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember
$ 345,032us-gaap_LoansReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerPortfolioSegmentMember
/ colb_LoansByMajorCategoryAxis
= colb_LoansExcludingPurchasedCreditImpairedLoansMember