Business Combinations - Additional Information (Detail)					3 Months Ended		6 Months Ended
	May. 18, 2015	Mar. 03, 2015 Branch	Sep. 05, 2014 USD ($) $ / shares shares	Nov. 25, 2013 USD ($) shares	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Assets					$ 5,077,483,000		$ 5,077,483,000		$ 4,853,320,000
Loans					3,295,471,000		3,295,471,000		3,156,652,000
Deposits					3,526,455,000		3,526,455,000		3,649,235,000
Cash paid to FNBW stockholders			$ 32,028,000
Common stock shares issued | shares			1,357,983
Stock split conversion ratio	3
Transaction value, cash and stock			$ 64,936,000
Goodwill resulting from acquisition of FNBW					48,987,000		48,987,000		48,651,000
Alliance [Member]
Business Acquisition [Line Items]
Assets					415,000,000		415,000,000		421,000,000
Loans					315,000,000		315,000,000		310,000,000
Deposits					338,000,000		338,000,000		345,000,000
Number of branch locations | Branch		8
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Cash paid to FNBW stockholders				$ 32,000,000
Common stock shares issued | shares				1,357,983
Stock split conversion ratio				3
Goodwill resulting from acquisition of FNBW			16,706,000		16,706,000		16,706,000		$ 16,370,000
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] | Acquisition Related Costs [Member]
Business Acquisition [Line Items]
Integration expenses					$ 62,000	$ 157,000	$ 97,000	$ 412,000
First Wyoming Financial Corporation [Member] | NASDAQ Stock Market [Member]
Business Acquisition [Line Items]
Closing share price | $ / shares			$ 24.23