Loans - Schedule of Loans Acquired Through Merger in Accordance with FASB ASC 310-30 (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Sep. 05, 2014
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required principal and interest at acquisition	$ 5,200
First Wyoming Financial Corporation [Member]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required principal and interest at acquisition		$ 27,086
Contractual cash flows not expected to be collected (nonaccretable difference)		7,956
Expected cash flows at acquisition		19,130
Interest component of expected cash flows (accretable yield)		1,790
Fair value of acquired loans accounted for under FASB ASC 310-30		$ 17,340