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Cash and Cash Equivalents (Details) (Federal Reserve Bank [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Federal Reserve Bank [Member]
   
Restricted Cash and Cash Equivalents Items [Line Items]    
Restricted Cash and Cash Equivalents $ 47.4us-gaap_RestrictedCashAndCashEquivalents
/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis
= colb_FederalReserveBankMember
$ 36.3us-gaap_RestrictedCashAndCashEquivalents
/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis
= colb_FederalReserveBankMember