Condensed Financial Information - Parent Company Only - Condensed Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INCOME
Other income	$ 3,123	$ 1,769	$ 800
EXPENSE
Noninterest expenses	104,534	99,917	87,211
Income taxes (benefit) provision	27,469	21,321	16,167
NET INCOME AVAILABLE TO COMMON SHAREHOLDERS	52,580	41,518	32,481
Parent Company
INCOME
Dividends from subsidiaries	26,665	20,687	23,376
Other income	2,750	199	1,987
TOTAL INCOME	29,415	20,886	25,363
EXPENSE
Interest on subordinated debentures	5,091	2,171	1,009
Noninterest expenses	377	442	605
TOTAL EXPENSE	5,468	2,613	1,614
Income before (benefit) provision for income taxes	23,947	18,273	23,749
Income taxes (benefit) provision	(2,018)	(845)	67
Income before equity in undistributed income of subsidiaries	25,965	19,118	23,682
Equity in undistributed income of subsidiaries	26,615	22,400	8,799
NET INCOME AVAILABLE TO COMMON SHAREHOLDERS	$ 52,580	$ 41,518	$ 32,481