Goodwill and Intangible Assets (Schedule of Goodwill and Intangible Assets) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill and Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets (Excluding Goodwill)	$ 919	$ 919	$ 0
Intangible Assets, Net (Excluding Goodwill)	30,459	25,852	15,721
Goodwill and Intangible Assets [Roll Forward]
Total goodwill, beginning of period	343,952us-gaap_Goodwill	115,554us-gaap_Goodwill	115,554us-gaap_Goodwill
Established through acquisitions	38,585us-gaap_GoodwillAcquiredDuringPeriod	228,398us-gaap_GoodwillAcquiredDuringPeriod	0us-gaap_GoodwillAcquiredDuringPeriod
Total goodwill, end of period	382,537us-gaap_Goodwill	343,952us-gaap_Goodwill	115,554us-gaap_Goodwill
Core deposit intangible, net, beginning of period	25,852us-gaap_FiniteLivedIntangibleAssetsNet
CDI current period amortization	(6,293)us-gaap_AmortizationOfIntangibleAssets	(6,045)us-gaap_AmortizationOfIntangibleAssets	(4,445)us-gaap_AmortizationOfIntangibleAssets
Total core deposit intangible, end of period	30,459us-gaap_FiniteLivedIntangibleAssetsNet	25,852us-gaap_FiniteLivedIntangibleAssetsNet
Total goodwill and intangible assets, end of period	412,996colb_GoodwillAndIntangibleAssetsTotal	369,804colb_GoodwillAndIntangibleAssetsTotal	131,275colb_GoodwillAndIntangibleAssetsTotal
Core Deposits [Member]
Goodwill and Intangible Assets [Roll Forward]
Gross core deposit intangible balance, beginning of period	47,698	32,441	32,441
Accumulated amortization, beginning of period	(22,765)	(16,720)	(12,275)
Core deposit intangible, net, beginning of period	24,933	15,721	20,166
Established through acquisitions	10,900	15,257	0
CDI current period amortization	(6,293)	(6,045)	(4,445)
Total core deposit intangible, end of period	$ 29,540	$ 24,933	$ 15,721
Estimated life of CDI, in years	10 years

Finite Lived Intangible Assets Acquired During the Period

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Goodwill and intangible assets total

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The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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