Covered Assets and FDIC Loss-sharing Asset (FDIC Loss-sharing Asset) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Balance at beginning of period	$ 15,174us-gaap_FDICIndemnificationAsset	$ 39,846us-gaap_FDICIndemnificationAsset
Cash received from the FDIC	(659)	1,678
FDIC reimbursable losses, net	(21)	132
Amortization, net	(2,294)	(6,452)
Impairment	1,532	1,938
Sale of other real estate	(420)colb_FdicLossSharingAssetDisposals	(756)colb_FdicLossSharingAssetDisposals
Write-downs of other real estate	1,071	516
Other	261colb_FdicLossSharingAssetOther	(65)colb_FdicLossSharingAssetOther
Balance at end of period	14,644us-gaap_FDICIndemnificationAsset	36,837us-gaap_FDICIndemnificationAsset
Amount of Estimated Clawback Liability	4,199colb_AmountOfEstimatedClawbackLiability
Non-Single Family Covered Assets	160,668colb_NonSingleFamilyCoveredAssets
Single Family Covered Assets	45,186colb_SingleFamilyCoveredAssets
Columbia River Bank [Member]
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Balance at end of period	2,124
Amount of Estimated Clawback Liability	4,063
Non-Single Family Covered Assets	107,135
Single Family Covered Assets	9,994
American Marine Bank [Member]
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Balance at end of period	4,824
Amount of Estimated Clawback Liability	136
Non-Single Family Covered Assets	16,914
Single Family Covered Assets	26,391
Summit Bank [Member]
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Balance at end of period	4,162
Amount of Estimated Clawback Liability	0
Non-Single Family Covered Assets	13,856
Single Family Covered Assets	6,322
First Heritage Bank [Member]
Covered Assets And FDIC Loss Sharing Asset [Line Items]
Balance at end of period	3,534
Amount of Estimated Clawback Liability	0
Non-Single Family Covered Assets	22,763
Single Family Covered Assets	$ 2,479