Business Combination (Details) $ in Thousands			3 Months Ended
	Apr. 06, 2016	Jul. 24, 2015	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Assets			$ 8,690,680		$ 8,728,196
Loans and Leases Receivable, Net Amount			5,164,534		5,169,379
Deposits			7,107,463		7,088,937
Goodwill			204,481		$ 204,523
Business Combination, Acquisition Related Costs			0	$ 70
Flathead Bank [Member]
Business Acquisition [Line Items]
Estimate of Payments to Acquire Businesses, Gross			34,237
Assets			$ 231,574
United Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Jul. 24, 2015
Number of Subsidiary Banks			1
Goodwill, Purchase Accounting Adjustments			$ 42
Subsequent Event [Member] | United Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Apr. 06, 2016