Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Income Tax Contingency [Line Items]
Federal operating loss carryforwards, set to begin to expire in 2024	$ 18,400									$ 18,400
Idaho state operating loss carryforwards, set to begin to expire in 2024	27,600									27,600
Interest and penalties on unrecognized tax benefits										0
State operating loss carryforwards, set to begin to expire in 2024	170									170
Unrecognized tax position	0									0
Federal tax credit carryforwards	404									404
State tax credit carryforwards, set to expire in 2015	560									560
Current tax (benefit) expense										43,069	$ 36,426		$ 21,565
Deferred tax expense (benefit)										1,846	6,367		14,646
Income tax provision	12,317	$ 12,124	$ 11,245	$ 9,229	$ 10,598		$ 11,515	$ 9,853	$ 10,827	44,915	42,793		36,211
Deferred tax assets:
Allowance for loan and lease losses	26,638				26,024					26,638	26,024
Supplemental executive retirement plan	16,232				14,716	[1]				16,232	14,716	[1]
Stock option and restricted stock	1,922				1,423					1,922	1,423
OREO	111				813					111	813
Nonaccrual interest	320				69					320	69
Deferred tax assets, purchase accounting	2,613				9,457					2,613	9,457
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross	7,492				0	[1]				7,492	0	[1]
Deferred Tax Assets, Operating Loss Carryforwards	8,597				11,467					8,597	11,467
Other	(851)				(180)					(851)	(180)
Total deferred tax assets	65,835				64,149					65,835	64,149
Deferred tax liabilities:
Asset purchase tax basis difference	(9,037)				(9,058)					(9,037)	(9,058)
FHLB stock dividends	(1,232)				(1,232)					(1,232)	(1,232)
Deferred loan fees	(5,126)				(5,202)					(5,126)	(5,202)
deferred tax liabilities, unrealized gains on available-for-sale securities	0				30	[1]				0	30	[1]
Depreciation	0				(3,730)					0	(3,730)
Deferred Tax Liabilities, Other	(155)				0					(155)	0
Total deferred tax liabilities	15,550				19,252					15,550	19,252
Deferred Tax Assets, Property, Plant and Equipment	1,059				0					1,059	0
Net deferred tax (liability) asset	50,285				44,897					50,285	44,897
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate										$ 52,424	$ 49,567		$ 41,225
Income tax based on statutory rate, percent										35.00%	35.00%		35.00%
Tax exempt instrument										$ (7,433)	$ (6,761)		$ (5,328)
Tax exempt instrument, percent										(5.00%)	(5.00%)		(5.00%)
Life insurance proceeds										$ (1,680)	$ (1,554)		$ (1,352)
Life insurance proceeds, percent										(1.00%)	(1.00%)		(1.00%)
effective income tax rate reconciliation, nondeductible expense, business combination, amount										$ 0	$ 0		$ 448
effective income tax rate reconciliation, nondeductible expense, business combination, percent										0.00%	0.00%		0.00%
Other, net										$ 1,604	$ 1,541		$ 1,218
Other, net, percent										1.00%	1.00%		2.00%
Income tax provision	$ 12,317	$ 12,124	$ 11,245	$ 9,229	$ 10,598		$ 11,515	$ 9,853	$ 10,827	$ 44,915	$ 42,793		$ 36,211
Income tax provision (benefit), percent										30.00%	30.00%		31.00%

[1] Reclassified to conform to the current period’s presentation.