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