Acquisition - Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill resulting from acquisition	$ 171,565,000	$ 139,971,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	18,653,000
Securities available-for-sale	64,501,000
Loans	266,327,000
Identifiable intangible assets	3,167,000
Other assets	15,375,000
Total identifiable assets acquired	368,023,000
Deposits	341,902,000
Other liabilities	(373,000)
Total liabilities assumed	341,529,000
Fair value of net identifiable assets acquired	26,494,000
Goodwill resulting from acquisition	$ 31,593,000