Business Combinations - Additional Information (Detail)				3 Months Ended		9 Months Ended
	May. 18, 2015	Sep. 05, 2014 USD ($) $ / shares shares	Nov. 25, 2013 USD ($) shares	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Stock split conversion ratio	3
Goodwill resulting from acquisition of FNBW				$ 48,787,000		$ 48,787,000		$ 48,651,000
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Cash paid to FNBW stockholders		$ 32,028,000	$ 32,000,000
Common stock shares issued | shares		1,357,983	1,357,983
Stock split conversion ratio			3
Transaction value, cash and stock		$ 64,936,000
Goodwill resulting from acquisition of FNBW		16,506,000		16,506,000		16,506,000		$ 16,370,000
First Wyoming Financial Corporation [Member] | Acquisition Related Costs [Member]
Business Acquisition [Line Items]
Merger expenses				$ 12,000.0	$ 2,500,000	$ 109,000.0	$ 2,900,000
First Wyoming Financial Corporation [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Core deposit intangibles		$ 3,200,000
Amortized period		10 years
First Wyoming Financial Corporation [Member] | NASDAQ Stock Market [Member]
Business Acquisition [Line Items]
Closing share price | $ / shares		$ 24.23