Business Combinations - Summary of Effect on Goodwill from Changes in Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Sep. 05, 2014
Business Acquisition [Line Items]
Goodwill, Beginning balance		$ 48,651
Goodwill, Ending balance	48,824	48,651
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Goodwill, Beginning balance		16,370	16,543
Goodwill, Ending balance	16,543	16,370	16,543
First Wyoming Financial Corporation [Member] | Assets [Member]
Business Acquisition [Line Items]
Effect of adjustments	$ 173