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Other Borrowings (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Nov. 01, 2014
Subordinated Borrowing [Line Items]    
Repayments of subordinated debt, Intermountain Statutory Trust I $ 8.3colb_RepaymentsofsubordinateddebtIntermountainStatutoryTrustI  
Subordinated Borrowing, Interest Rate 3.03%us-gaap_SubordinatedBorrowingInterestRate  
Intermountain Community Bancorp [Member]    
Subordinated Borrowing [Line Items]    
Business Combination, Purchase Price Allocation, Junior Subordinated Debentures   $ 16.5colb_BusinessCombinationPurchasePriceAllocationJuniorSubordinatedDebentures
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember