Condensed Financial Information - Parent Company Only - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash and due from banks	$ 142,933	$ 175,801	$ 118,493	$ 109,316
Investment securities, available for sale	646,135	606,704
Investment securities, held to maturity	139,685	147,614
Other assets	35,576	30,550
TOTAL ASSETS	5,405,639	5,093,131
LIABILITIES AND STOCKHOLDERS’ EQUITY
Other liabilities	31,920	28,248
Subordinated debentures	104,902	104,784
Total stockholders’ equity	583,122	550,044	400,516	379,438
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	5,405,639	5,093,131
Parent Company
ASSETS
Cash and due from banks	17,695	11,675	$ 15,921	$ 11,893
Investment securities, available for sale	5,158	7,757
Investment securities, held to maturity	1,000	1,000
Investment in subsidiaries	659,180	631,500
Other assets	6,013	5,018
TOTAL ASSETS	689,046	656,950
LIABILITIES AND STOCKHOLDERS’ EQUITY
Other liabilities	1,022	2,122
Subordinated debentures	104,902	104,784
Total stockholders’ equity	583,122	550,044
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 689,046	$ 656,950