Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Federal operating loss carryforwards, set to begin to expire in 2024	$ 23,600								$ 23,600
Idaho state operating loss carryforwards, set to begin to expire in 2024	32,700								32,700
Interest and penalties on unrecognized tax benefits									0
State operating loss carryforwards, set to begin to expire in 2024	540								540
Unrecognized tax position	0								0
Federal tax credit carryforwards	569								569
State tax credit carryforwards, set to expire in 2015	1,500								1,500
Current tax (benefit) expense									36,426	$ 21,565	$ 21,581
Deferred tax expense (benefit)									6,367	14,646	5,413
Provision for income taxes	10,598	$ 11,515	$ 9,853	$ 10,827	$ 9,167	$ 9,605	$ 8,643	$ 8,796	42,793	36,211	26,994
Deferred tax assets:
Allowance for loan and lease losses	26,024				26,341				26,024	26,341
Supplemental executive retirement plan	10,770				9,037				10,770	9,037
Stock option and restricted stock	1,423				1,177				1,423	1,177
OREO	813				1,101				813	1,101
Nonaccrual interest	69				68				69	68
Deferred tax assets, purchase accounting	9,457				15,272				9,457	15,272
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross	3,916				0				3,916	0
Deferred Tax Assets, Operating Loss Carryforwards	11,467				14,929				11,467	14,929
Other	180				532				180	532
Total deferred tax assets	64,119				68,457				64,119	68,457
Deferred tax liabilities:
Asset purchase tax basis difference	(9,058)				(6,595)				(9,058)	(6,595)
FHLB stock dividends	(1,232)				(4,086)				(1,232)	(4,086)
Deferred loan fees	(5,202)				(4,691)				(5,202)	(4,691)
deferred tax liabilities, unrealized gains on available-for-sale securities	0				2,987				0	2,987
Depreciation	(3,730)				(5,394)				(3,730)	(5,394)
Total deferred tax liabilities	19,222				23,753				19,222	23,753
Net deferred tax (liability) asset	44,897				44,704				44,897	44,704
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate									$ 49,567	$ 41,225	$ 30,454
Income tax based on statutory rate, percent									35.00%	35.00%	35.00%
Tax exempt instrument									$ (6,761)	$ (5,328)	$ (4,113)
Tax exempt instrument, percent									(5.00%)	(5.00%)	(5.00%)
Life insurance proceeds									$ (1,554)	$ (1,352)	$ (1,250)
Life insurance proceeds, percent									(1.00%)	(1.00%)	(1.00%)
effective income tax rate reconciliation, nondeductible expense, business combination, amount									$ 0	$ 448	$ 1,362
effective income tax rate reconciliation, nondeductible expense, business combination, percent									0.00%	0.00%	2.00%
Other, net									$ 1,541	$ 1,218	$ 541
Other, net, percent									1.00%	2.00%	0.00%
Provision for income taxes	$ 10,598	$ 11,515	$ 9,853	$ 10,827	$ 9,167	$ 9,605	$ 8,643	$ 8,796	$ 42,793	$ 36,211	$ 26,994
Income tax provision (benefit), percent									30.00%	31.00%	31.00%