Business Combinations - Summary of Effect on Goodwill from Changes in Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014	Sep. 30, 2014	Sep. 05, 2014
Business Acquisition [Line Items]
Goodwill, Beginning balance			$ 32,235	$ 28,146
Goodwill, Ending balance	48,651	48,651	32,235	28,146
Array Financial Group, Inc. [Member]
Business Acquisition [Line Items]
Goodwill, Beginning balance			4,089		4,135
Goodwill, Ending balance	4,135	4,135	4,089		4,135
Array Financial Group, Inc. [Member] | Other Assets [Member]
Business Acquisition [Line Items]
Effect of adjustments	(338)
Array Financial Group, Inc. [Member] | Contingent Liabilities [Member]
Business Acquisition [Line Items]
Effect of adjustments	181
Array Financial Group, Inc. [Member] | Other Liabilities [Member]
Business Acquisition [Line Items]
Effect of adjustments	203
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Goodwill, Beginning balance						16,467	16,370
Goodwill, Ending balance	16,370	16,370				16,467	16,370
First Wyoming Financial Corporation [Member] | Final Purchase Price [Member]
Business Acquisition [Line Items]
Effect of adjustments		(9)
First Wyoming Financial Corporation [Member] | Assets [Member]
Business Acquisition [Line Items]
Effect of adjustments		(187)
First Wyoming Financial Corporation [Member] | Liability [Member]
Business Acquisition [Line Items]
Effect of adjustments		$ 99