v2.4.1.9

Securities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	$ 57.2us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds	$ 6.4us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In Unrealized Loss Position	77us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Number Of Securities In Continuous Loss Position For Twelve Months or More	34us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_USGovernmentAgencyAndGovernmentSponsoredEnterpriseMortgageBackedSecuritiesAndCollateralizedMortgageObligationsMember
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In Unrealized Loss Position	66us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
Number Of Securities In Continuous Loss Position For Twelve Months or More	32us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In Unrealized Loss Position	18us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Number Of Securities In Continuous Loss Position For Twelve Months or More	14us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
US Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In Unrealized Loss Position	4us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Number Of Securities In Continuous Loss Position For Twelve Months or More	2us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number Of Securities In Unrealized Loss Position	1us-gaap_AvailableForSaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsLessThanOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember
Number Of Securities In Continuous Loss Position For Twelve Months or More	1us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = colb_OtherSecuritiesMember

X

- Definition

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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- Definition

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for less than one year or normal operating cycle, if longer, for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

No definition available.

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