Business Combination (Details)					3 Months Ended		9 Months Ended
		Jul. 24, 2015 USD ($)	Mar. 26, 2015	Jul. 31, 2014	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Assets					$ 8,604,530,000		$ 8,604,530,000		$ 8,609,936,000
Loans and Leases Receivable, Net Amount					5,102,224,000		5,102,224,000		4,823,243,000
Deposits					7,035,794,000		7,035,794,000		7,006,212,000
Finite-lived Intangible Assets Acquired		$ 695
Finite-Lived Intangible Asset, Useful Life		10 years
Goodwill					204,409,000		204,409,000		$ 205,574,000
Business Combination, Acquisition Related Costs					$ 566,000	$ 1,052,000	629,000	$ 1,649,000
United Bank Acquisition, as recorded by Absarokee [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		$ 5,598,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities		27,574,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable		37,520,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable, Allowance		(899,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		2,851,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets		816,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		73,460,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		63,565,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Securities Under Agreement to Repurchase		2,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		1,222,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Liabilities		79,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		66,866,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		6,594,000
United Bank Acquisition, Fair Value Adjustment [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable	[1]	(876,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable, Allowance	[2]	899,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	[3]	277,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	[4]	695,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets	[5]	82,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		1,077,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Securities Under Agreement to Repurchase		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	[6]	31,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Liabilities	[7]	440,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		471,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		606,000
United Bank Acquisition, as recorded by Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		5,598,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities		27,574,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable		36,644,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans and Leases Receivable, Allowance		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		3,128,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles		695,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Assets		898,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		74,537,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits		63,565,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Securities Under Agreement to Repurchase		2,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		1,253,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Liabilities		519,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		67,337,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		7,200,000
Payments to Acquire Businesses, Gross		7,234,000
Goodwill		$ 34,000
United Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement			Mar. 26, 2015
Estimate of Payments to Acquire Businesses, Gross							$ 7,234,000
Business Acquisition, Effective Date of Acquisition		Jul. 24, 2015
Number of Communities Adjacent to Existing Market Areas					3		3
Assets					$ 73,460,000		$ 73,460,000
Loans and Leases Receivable, Net Amount					37,520,000		37,520,000
Deposits					$ 63,565,000		$ 63,565,000
Number of Subsidiary Banks					1		1
Mountain West Bank
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition				Jul. 31, 2014
Goodwill, Purchase Accounting Adjustments							$ 1,199,000

[1] Write down of the book value of loans to their estimated fair values. The fair value of loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans.

[2] Adjustment to remove the Absarokee allowance for loan losses at acquisition date as the credit risk is accounted for in the fair value adjustment for loans receivable described in (1) above.

[3] Write up of the book value of premises and equipment to their estimated fair values based upon fair value estimates developed internally based upon comparable in-market transactions.

[4] ) Adjustment represents the value of the core deposit base assumed in the acquisition based upon an internal valuation based on analysis of recent acquisitions.

[5] ) Adjustment consists of a reduction in the value of accrued interest receivable, the write-off of pre-existing goodwill and recording the value of mortgage servicing assets acquired.

[6] Adjustment represents increase in the book value of Federal Home Loan Bank borrowings to their estimated fair market values based upon interest interest rates of similar advances with similar characteristics on the date of acquisition.

[7] Adjustment represents the net deferred tax liability resulting from fair value adjustments related to acquired assets, assumed liabilities, core deposit intangible assets and other purchase accounting adjustments.