Business Combinations - Schedule of Changes to Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill, Beginning balance	$ 48,651
Goodwill, Ending balance	48,787
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Goodwill, Beginning balance	16,370
Goodwill, Ending balance	16,506
First Wyoming Financial Corporation [Member] | Liability [Member]
Business Acquisition [Line Items]
Effect of adjustments	(100)
First Wyoming Financial Corporation [Member] | Assets [Member]
Business Acquisition [Line Items]
Effect of adjustments	$ 236