Business Combinations - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		0 Months Ended
	Sep. 05, 2014	Mar. 31, 2015	Mar. 31, 2014	Mar. 03, 2015 Branch	Nov. 25, 2013	Dec. 31, 2014
Business Acquisition [Line Items]
Assets		$ 4,946,480,000				$ 4,853,320,000
Loans		3,189,138,000				3,156,652,000
Deposits		3,522,278,000				3,649,235,000
Cash paid to FNBW stockholders	32,028,000
Common stock shares issued	452,661
Transaction value, cash and stock	64,936,000
Goodwill resulting from acquisition of FNBW		48,824,000				48,651,000
Equipment expense		2,082,000	1,687,000
Salary and benefits		21,010,000	18,474,000
Professional fees		1,472,000	1,106,000
Data processing expense		1,422,000	1,471,000
Alliance [Member]
Business Acquisition [Line Items]
Assets						421,000,000
Loans						310,000,000
Deposits						345,000,000
Number of branch locations				8
First Wyoming Financial Corporation [Member]
Business Acquisition [Line Items]
Cash paid to FNBW stockholders					32,000,000
Common stock shares issued					452,661
Goodwill resulting from acquisition of FNBW	16,543,000	16,543,000				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		35,000	247,000
Equipment expense		16,000
Salary and benefits		10,000
Professional fees		3,000	237,000
Data processing expense		$ 3,000
First Wyoming Financial Corporation [Member] | NASDAQ Stock Market [Member]
Business Acquisition [Line Items]
Closing share price	$ 72.70