Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Federal operating loss carryforwards, set to begin to expire in 2024	$ 24,800										$ 24,800
Idaho state operating loss carryforwards, set to begin to expire in 2024	26,900										26,900
Interest and penalties on unrecognized tax benefits											0	$ 0
State operating loss carryforwards, set to begin to expire in 2024	144										144
Unrecognized tax position	0						$ 0				0	0
Federal tax credit carryforwards	450										450
Current tax (benefit) expense											42,724	43,069	$ 36,426
Deferred tax expense (benefit)											22,431	1,846	6,367
Income tax provision	25,123	[1]	$ 18,338		$ 11,120	$ 10,574	12,317	$ 12,124	$ 11,245	$ 9,229	65,155	44,915	42,793
Deferred tax assets:
Allowance for loan and lease losses	18,315						26,638				18,315	26,638
Supplemental executive retirement plan	9,539						16,232				9,539	16,232
Stock option and restricted stock	1,438						1,922				1,438	1,922
OREO	521						111				521	111
Nonaccrual interest	163						320				163	320
Deferred tax assets, purchase accounting	0						2,613				0	2,613
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross	5,992						7,492				5,992	7,492
Deferred Tax Assets, Operating Loss Carryforwards	7,259						8,597				7,259	8,597
Other	(985)						(851)				(985)	(851)
Total deferred tax assets	44,212						65,835				44,212	65,835
Deferred tax liabilities:
Asset purchase tax basis difference	(5,709)						(9,037)				(5,709)	(9,037)
FHLB stock dividends	(782)						(1,232)				(782)	(1,232)
Deferred loan fees	(4,505)						(5,126)				(4,505)	(5,126)
Deferred tax liabilities, purchase accounting	(9,088)						0				(9,088)	0
Depreciation	(1,581)						0				(1,581)	0
Deferred Tax Liabilities, Other	(2,036)						(155)				(2,036)	(155)
Total deferred tax liabilities	23,701						15,550				23,701	15,550
Deferred Tax Assets, Property, Plant and Equipment	0						1,059				0	1,059
Net deferred tax (liability) asset	20,511						50,285				20,511	50,285
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate											$ 62,262	$ 52,424	$ 49,567
Income tax based on statutory rate, percent											35.00%	35.00%	35.00%
Federal income tax rate 2018											21.00%
Tax exempt instrument											$ (8,485)	$ (7,433)	$ (6,761)
Tax exempt instrument, percent											(5.00%)	(5.00%)	(5.00%)
Life insurance proceeds											$ (3,351)	$ (1,680)	$ (1,554)
Life insurance proceeds, percent											(2.00%)	(1.00%)	(1.00%)
effective income tax rate reconciliation, nondeductible expense, business combination, amount											$ 825	$ 0	$ 0
effective income tax rate reconciliation, nondeductible expense, business combination, percent											1.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount											$ 12,210	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent											7.00%	0.00%	0.00%
Other, net											$ 1,694	$ 1,604	$ 1,541
Other, net, percent											1.00%	1.00%	1.00%
Income tax provision	$ 25,123	[1]	$ 18,338		$ 11,120	$ 10,574	$ 12,317	$ 12,124	$ 11,245	$ 9,229	$ 65,155	$ 44,915	$ 42,793
Income tax provision (benefit), percent											37.00%	30.00%	30.00%

[1] During the fourth quarter of 2017, Columbia acquired Pacific Continental Corporation and also recorded a charge through provision for income taxes related to the re-measurement of our deferred tax assets pursuant to the newly enacted Tax Cuts and Jobs Act. See Note 2, “Business Combinations” for further information regarding this acquisition. See Note 24, “Income Tax” for further information regarding the re-measurement of our deferred tax assets.

[2] During the third quarter of 2017, Columbia sold its merchant card services portfolio. See Note 1, “Summary of Significant Accounting Policies” for further information regarding this transaction.