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Business combinations (Details) (USD $)
0 Months Ended 3 Months Ended
Apr. 01, 2013
Mar. 31, 2015
Mar. 31, 2014
Apr. 01, 2013
Business Acquisition [Line Items]        
Business Acquisition, Percentage of Voting Interests Acquired 100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired     100.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Business Acquisition, Pro Forma Net Income (Loss)     $ 21,078,000us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss  
West Coast Bancorp [Member]        
Business Acquisition [Line Items]        
Business Acquisition, Effective Date of Acquisition Nov. 01, 2014      
Business Acquisition, Cost of Acquired Entity, Purchase Price 131,900,000colb_BusinessCombinationCostofAcquiredEntityPurchasePriceTotal
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    131,900,000colb_BusinessCombinationCostofAcquiredEntityPurchasePriceTotal
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net 93,400,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    93,400,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Acquisition, Purchase Price Allocation, Liabilities, Deposits (736,800,000)colb_BusinessCombinationPurchasePriceAllocationLiabilitiesDeposits
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    (736,800,000)colb_BusinessCombinationPurchasePriceAllocationLiabilitiesDeposits
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Combination, Acquired Receivables, Fair Value 502,600,000us-gaap_BusinessCombinationAcquiredReceivablesFairValue
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    502,600,000us-gaap_BusinessCombinationAcquiredReceivablesFairValue
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Acquisition, Purchase Price Allocation, Amortizable Intangible Assets 10,900,000colb_BusinessCombinationPurchasePriceAllocationAmortizableIntangibleAssets
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    10,900,000colb_BusinessCombinationPurchasePriceAllocationAmortizableIntangibleAssets
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Acquisition, Purchase Price Allocation, Goodwill Amount 38,600,000colb_BusinessCombinationPurchasePriceAllocationGoodwillAmount
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
    38,600,000colb_BusinessCombinationPurchasePriceAllocationGoodwillAmount
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
Business Acquisition, Pro Forma Revenue     97,488,000us-gaap_BusinessAcquisitionsProFormaRevenue
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
 
Business Acquisition, Pro Forma Earnings Per Share, Basic     $ 0.38us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
 
Business Acquisition, Pro Forma Earnings Per Share, Diluted     $ 0.37us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
 
Business Combination, Acquisition Related Costs   72,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
966,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_WestCoastBancorpMember
 
Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   2,902,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
   
Compensation and employee benefits [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   273,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_CompensationandemployeebenefitsMember
   
Occupancy [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   499,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_OccupancyMember
   
Advertising and promotion [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   96,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_AdvertisingandpromotionMember
   
Data processing and communications [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   1,558,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_DataprocessingandcommunicationsMember
   
Legal and professional fees [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   385,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_LegalandprofessionalfeesMember
   
Other noninterest expenses [Member] | Intermountain Community Bancorp [Member]        
Business Acquisition [Line Items]        
Business Combination, Acquisition Related Costs   $ 91,000us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= colb_IntermountainCommunityBancorpMember
/ us-gaap_IncomeStatementLocationAxis
= colb_OthernoninterestexpensesMember