Income Tax (Details) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
Income Tax Contingency [Line Items]
Unrecognized tax position	$ 0									$ 0
Interest and penalties on unrecognized tax benefits										0
Federal operating loss carryforwards, set to begin to expire in 2024	30,700,000									30,700,000
Idaho state operating loss carryforwards, set to begin to expire in 2024	38,100,000									38,100,000
Deferred Tax Assets, Operating Loss Carryforwards	14,929,000				1,250,000	[1]				14,929,000	1,250,000	[1]
State operating loss carryforwards, set to begin to expire in 2024	13,500,000									13,500,000
Federal tax credit carryforwards	561,000									561,000
State tax credit carryforwards, set to expire in 2015	1,600,000									1,600,000
Current tax (benefit) expense										21,565,000	21,581,000		21,218,000
Deferred tax expense (benefit)										14,646,000	5,413,000		(3,656,000)
Income tax expense (benefit)	9,167,000	9,605,000	8,643,000	8,796,000	7,911,000		6,734,000	7,414,000	4,935,000	36,211,000	26,994,000		17,562,000
Deferred tax assets:
Allowance for loan and lease losses	26,341,000				27,196,000					26,341,000	27,196,000
Supplemental executive retirement plan	9,037,000				8,565,000					9,037,000	8,565,000
Stock option and restricted stock	1,177,000				917,000					1,177,000	917,000
OREO	1,101,000				7,929,000					1,101,000	7,929,000
Nonaccrual interest	68,000				2,354,000					68,000	2,354,000
Deferred tax assets, purchase accounting	15,272,000				15,551,000					15,272,000	15,551,000
Deferred Tax Assets, Unrealized Losses on Available-for-Sale Securities, Gross	0				7,176,000					0	7,176,000
deferred tax liabilities, unrealized gains on available-for-sale securities	2,987,000				0					2,987,000	0
Other	532,000				491,000	[1]				532,000	491,000	[1]
Total deferred tax assets	68,457,000				71,429,000					68,457,000	71,429,000
Deferred tax liabilities:
Asset purchase tax basis difference	(6,595,000)				(7,754,000)					(6,595,000)	(7,754,000)
FHLB stock dividends	(4,086,000)				(4,159,000)					(4,086,000)	(4,159,000)
Deferred loan fees	(4,691,000)				(4,512,000)					(4,691,000)	(4,512,000)
Depreciation	(5,394,000)				(7,076,000)					(5,394,000)	(7,076,000)
Total deferred tax liabilities	23,753,000				23,501,000					23,753,000	23,501,000
Net deferred tax (liability) asset	44,704,000				47,928,000					44,704,000	47,928,000
Reconciliation of effective income tax rate with federal statutory tax rate
Income tax based on statutory rate										41,225,000	30,454,000		22,297,000
Income tax based on statutory rate, percent										35.00%	35.00%		35.00%
Tax exempt instrument										(5,328,000)	(4,113,000)		(3,906,000)
Tax exempt instrument, percent										(5.00%)	(5.00%)		(6.00%)
Life insurance proceeds										(1,352,000)	(1,250,000)		(1,001,000)
Life insurance proceeds, percent										(1.00%)	(1.00%)		(2.00%)
effective income tax rate reconciliation, nondeductible expense, business combination, amount										448,000	1,362,000		0
effective income tax rate reconciliation, nondeductible expense, business combination, percent										0.00%	2.00%		0.00%
Other, net										1,218,000	541,000		172,000
Other, net, percent										2.00%	0.00%		0.00%
Income tax expense (benefit)	$ 9,167,000	$ 9,605,000	$ 8,643,000	$ 8,796,000	$ 7,911,000		$ 6,734,000	$ 7,414,000	$ 4,935,000	$ 36,211,000	$ 26,994,000		$ 17,562,000
Income tax provision (benefit), percent										31.00%	31.00%		27.00%

[1] Reclassified to conform to current period presentation. The reclassification was limited to including previously reported amounts for net operating losses and credit carryforwards, which had been included in the row for other deferred tax assets into a separate row.