Business Combinations - Summary of Consideration Paid and Fair Value of Identifiable Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Oct. 09, 2015	Sep. 05, 2014	Nov. 25, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liabilities assumed:
Goodwill resulting from acquisition				$ 85,212	$ 48,651	$ 32,235
First Wyoming Financial Corporation [Member]
Consideration Paid:
Common shares issued (2,459,120)		$ 32,908
Cash paid to Alliance stockholders		32,028	$ 32,000
Value of consideration		64,936
Assets acquired:
Cash and due from banks		40,605
Investment securities		41,822
Loans		175,895
Premises and equipment		1,611
Deferred income taxes		3,216			3,100
Bank owned life insurance		12,576
Core deposit intangible		3,240			3,200
Other real estate owned		1,593
Other assets		4,659
Total assets		285,217
Liabilities assumed:
Deposits		228,844
FHLB advances		5,052
Other liabilities		2,891
Total liabilities		236,787
Net assets acquired		48,430
Goodwill resulting from acquisition		$ 16,506		16,506	$ 16,370
Alliance Bancorp, Inc. [Member]
Consideration Paid:
Common shares issued (2,459,120)	$ 71,345
Cash paid to Alliance stockholders	26,576
Value of consideration	97,921
Assets acquired:
Cash and due from banks	67,439
Investment securities	3,002
Loans	307,695
Premises and equipment	2,685
Deferred income taxes	7,544
Bank owned life insurance	12,923
Core deposit intangible	2,635			$ 3,100
Other real estate owned	768
Other assets	2,986
Total assets	407,677
Liabilities assumed:
Deposits	341,682
Other borrowings	2,826
Other liabilities	1,673
Total liabilities	346,181
Net assets acquired	61,496
Goodwill resulting from acquisition	$ 36,425