Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	[2]	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Current Federal Tax Expense (Benefit)											$ 33,400	$ 39,708	$ 41,365
Federal operating loss carryforwards, set to begin to expire in 2024	$ 20,200										20,200
Idaho state operating loss carryforwards, set to begin to expire in 2024	18,700										18,700
Interest and penalties on unrecognized tax benefits											0	0
State operating loss carryforwards, set to begin to expire in 2024	132										132
Unrecognized tax position	0				$ 0						0	0
Current State and Local Tax Expense (Benefit)											5,446	3,016	1,704
Current tax (benefit) expense											38,846	42,724	43,069
Deferred State and Local Income Tax Expense (Benefit)											399	907	1,296
Deferred Federal Income Tax Expense (Benefit)											(291)	21,524	550
Deferred tax expense (benefit)											108	22,431	1,846
Income tax provision	10,734	$ 11,406	$ 9,999	$ 6,815	25,123	[1]	$ 18,338		$ 11,120	$ 10,574	38,954	65,155	44,915
Deferred tax assets:
Allowance for loan and lease losses	20,578				18,315						20,578	18,315
Supplemental executive retirement plan	9,501				9,539						9,501	9,539
Stock option and restricted stock	1,850				1,438						1,850	1,438
OREO	288				521						288	521
Nonaccrual interest	446				163						446	163
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross	10,129				5,992						10,129	5,992
Deferred Tax Assets, Operating Loss Carryforwards	5,356				7,259						5,356	7,259
Other	(733)				(985)						(733)	(985)
Total deferred tax assets	48,881				44,212						48,881	44,212
Deferred tax liabilities:
Asset purchase tax basis difference	(7,229)				(5,709)						(7,229)	(5,709)
FHLB stock dividends	(790)				(782)						(790)	(782)
Deferred loan fees	(4,399)				(4,505)						(4,399)	(4,505)
Deferred tax liabilities, purchase accounting	(9,245)				(9,088)						(9,245)	(9,088)
Depreciation	(2,609)				(1,581)						(2,609)	(1,581)
Deferred Tax Liabilities, Other	(195)				(2,036)						(195)	(2,036)
Total deferred tax liabilities	24,467				23,701						24,467	23,701
Net deferred tax (liability) asset	24,414				20,511						24,414	20,511
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate											$ 44,485	$ 62,262	$ 52,424
Income tax based on statutory rate, percent											21.00%	35.00%	35.00%
Federal income tax rate 2018											21.00%
Tax exempt instrument											$ (6,423)	$ (8,485)	$ (7,433)
Tax exempt instrument, percent											(3.00%)	(5.00%)	(5.00%)
Life insurance proceeds											$ (1,261)	$ (3,351)	$ (1,680)
Life insurance proceeds, percent											(1.00%)	(2.00%)	(1.00%)
effective income tax rate reconciliation, nondeductible expense, business combination, amount											$ 0	$ 825	$ 0
effective income tax rate reconciliation, nondeductible expense, business combination, percent											0.00%	1.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount											$ 0	$ 12,210	$ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent											0.00%	7.00%	0.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount											$ 4,931	$ 2,550	$ 1,950
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent											2.00%	1.00%	1.00%
Other, net											$ (2,778)	$ (856)	$ (346)
Other, net, percent											(1.00%)	0.00%	0.00%
Income tax provision	$ 10,734	$ 11,406	$ 9,999	$ 6,815	$ 25,123	[1]	$ 18,338		$ 11,120	$ 10,574	$ 38,954	$ 65,155	$ 44,915
Income tax provision (benefit), percent											18.00%	37.00%	30.00%

[1] During the fourth quarter of 2017, Columbia acquired Pacific Continental 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 23, “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.