Business Combination (Details) $ in Thousands				6 Months Ended
	Jul. 24, 2015	Mar. 26, 2015	Jul. 31, 2014	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Assets				$ 8,386,218	$ 8,609,936
Loans and Leases Receivable, Net Amount				5,027,394	4,823,243
Deposits				6,804,401	$ 7,006,212
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
Number of Communities Adjacent to Existing Market Areas				3
Assets				$ 73,272
Loans and Leases Receivable, Net Amount				37,520
Deposits				$ 63,580
Number of Subsidiary Banks				1
Mountain West Bank
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition			Jul. 31, 2014
Goodwill, Purchase Accounting Adjustments				$ 1,199
Subsequent Event [Member] | United Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Jul. 24, 2015