v2.4.1.9

Securities (Securities Available for Sale) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	$ 1,981,977us-gaap_AvailableForSaleSecuritiesAmortizedCost	$ 2,087,069us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available-for-sale Debt Securities Gross Unrealized Gain, instant	31,790us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	25,527us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(6,608)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	(14,339)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Securities available for sale	2,007,159us-gaap_AvailableForSaleSecurities	2,098,257us-gaap_AvailableForSaleSecurities
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	1,093,576us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember	1,160,378us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Available-for-sale Debt Securities Gross Unrealized Gain, instant	13,765us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember	10,219us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(4,641)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember	(8,210)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Securities available for sale	1,102,700us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember	1,162,387us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
State and Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	486,969us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember	483,578us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Available-for-sale Debt Securities Gross Unrealized Gain, instant	15,434us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember	14,432us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(732)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember	(1,526)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Securities available for sale	501,671us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember	496,484us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	375,230us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember	416,919us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Available-for-sale Debt Securities Gross Unrealized Gain, instant	2,551us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember	856us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(983)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember	(4,069)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Securities available for sale	376,798us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember	413,706us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	20,918us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	20,910us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Available-for-sale Debt Securities Gross Unrealized Gain, instant	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(140)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	(411)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Securities available for sale	20,778us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember	20,499us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, amortized cost	5,284us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember	5,284us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember
Available-for-sale Debt Securities Gross Unrealized Gain, instant	40us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember	20us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(112)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember	(123)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember
Securities available for sale	$ 5,212us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember	$ 5,181us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember

X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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