Business Combination (Details) $ in Thousands			3 Months Ended	6 Months Ended
	Apr. 06, 2016	Jul. 24, 2015	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Assets				$ 8,605,376	$ 8,728,196
Flathead Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Apr. 06, 2016
Assets					$ 231,574
Estimate of Payments to Acquire Businesses, Gross			$ 34,237
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