Acquisitions and Asset Purchase - Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill resulting from acquisition	$ 139,971,000	$ 139,971,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	65,197,000
Securities available-for-sale	42,903,000
Loans	248,380,000
Identifiable intangible assets	2,343,000
Other assets	15,262,000
Total identifiable assets acquired	374,085,000
Deposits	343,583,000
Subordinated debt	13,125,000
Other liabilities	1,651,000
Total liabilities assumed	358,359,000
Fair value of net identifiable assets acquired	15,726,000
Goodwill resulting from acquisition	$ 43,922,000