v2.4.1.9

Other Real Estate Owned (Summary of Other Real Estate Owned) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Other Real Estate Owned [Line Items]
Balance, beginning of period	$ 22,190,000us-gaap_RealEstateAcquiredThroughForeclosure	$ 35,927,000us-gaap_RealEstateAcquiredThroughForeclosure
Transfers in	4,692,000colb_RealEstateAcquiredThroughForeclosureTransfersInNetOfWriteDowns	5,751,000colb_RealEstateAcquiredThroughForeclosureTransfersInNetOfWriteDowns
Valuation adjustments	(197,000)colb_TransferOtherRealEstateWriteDown	(1,580,000)colb_TransferOtherRealEstateWriteDown
Proceeds from Sale of OREO Property	5,122,000us-gaap_ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans	11,205,000us-gaap_ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans
Gain (loss) on sale of OREO	1,736,000us-gaap_GainsLossesOnSalesOfOtherRealEstate	1,659,000us-gaap_GainsLossesOnSalesOfOtherRealEstate
Total OREO, end of period	23,299,000us-gaap_RealEstateAcquiredThroughForeclosure	30,552,000us-gaap_RealEstateAcquiredThroughForeclosure
Mortgage Loans in Process of Foreclosure, Amount	2,700,000colb_MortgageLoansinProcessofForeclosureAmount
One-to-Four Family Residential [Member]
Other Real Estate Owned [Line Items]
Total OREO, end of period	$ 2,100,000us-gaap_RealEstateAcquiredThroughForeclosure / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = colb_OneToFourFamilyResidentialMember

X

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

No definition available.

+ Details

Name:	colb_MortgageLoansinProcessofForeclosureAmount
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	colb_OtherRealEstateOwnedLineItems
Namespace Prefix:	colb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Real Estate Acquired Through Foreclosure Transfers In Net Of Write Downs

+ References

No definition available.

+ Details

Name:	colb_RealEstateAcquiredThroughForeclosureTransfersInNetOfWriteDowns
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Transfer Other Real Estate Write Down

+ References

No definition available.

+ Details

Name:	colb_TransferOtherRealEstateWriteDown
Namespace Prefix:	colb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The net gain (loss) resulting from sales and other disposals of other real estate owned, increases (decreases) in the valuation allowance for foreclosed real estate, and write-downs of other real estate owned after acquisition or physical possession.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14(d))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 14

-Subparagraph d

-Article 9

+ Details

Name:	us-gaap_GainsLossesOnSalesOfOtherRealEstate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The cash inflow from proceeds From Sales of Wholly Owned Real Estate and Real Estate Acquired in Settlement of Loans.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfWhollyOwnedRealEstateAndRealEstateAcquiredInSettlementOfLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=colb_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: