Condensed Financial Information - Parent Company Only - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	$ 52,580	$ 41,518	$ 32,481
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Gain on early extinguishment of debt	0	0	(1,830)
Excess tax benefits	587	0	0
(Increase) decrease in other assets	(25,065)	(5,600)	(5,257)
(Decrease) increase in other liabilities	3,705	1,618	3,691
NET CASH PROVIDED BY OPERATING ACTIVITIES	67,466	50,073	40,766
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of available for sale securities	(140,565)	(245,699)	(92,904)
Purchases of held to maturity securities	(35,841)	(59,715)	(33,811)
Proceeds from sale of available for sale securities	4,500	15,654	33,613
NET CASH USED IN INVESTING ACTIVITIES	(355,149)	(442,754)	(324,937)
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(18,853)	(16,007)	(12,586)
Proceeds from issuance of common stock, net	0	48,678	22
Proceeds from issuance of subordinated debt, net	0	73,516	0
Redemption of subordinated debentures, net	0	0	(8,170)
Retirement of restricted stock	(773)	(206)	(254)
Excess tax benefits	0	43	59
Exercise of stock options	321	285	124
NET CASH PROVIDED BY FINANCING ACTIVITIES	254,815	449,989	293,348
Net increase (decrease) in cash and cash equivalents	(32,868)	57,308	9,177
Cash and cash equivalents, beginning of year	175,801	118,493	109,316
CASH AND CASH EQUIVALENTS, END OF YEAR	142,933	175,801	118,493
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	52,580	41,518	32,481
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Gain on securities	(2,539)	0	(29)
Amortization of subordinated debt costs	118	30	0
Gain on early extinguishment of debt	0	0	(1,830)
Excess tax benefits	587	0	0
(Increase) decrease in other assets	(1,927)	(922)	3,861
(Decrease) increase in other liabilities	(17)	1,010	176
Equity in undistributed income of subsidiaries	(26,615)	(22,400)	(8,799)
NET CASH PROVIDED BY OPERATING ACTIVITIES	22,187	19,236	25,860
CASH FLOWS FROM INVESTING ACTIVITIES
Net cash used in acquisition	0	(5,356)	0
Purchases of available for sale securities	(79)	(62)	(56)
Purchases of held to maturity securities	0	0	(1,000)
Proceeds from sale of available for sale securities	3,217	0	29
Contribution to subsidiary	0	(124,373)	0
NET CASH USED IN INVESTING ACTIVITIES	3,138	(129,791)	(1,027)
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(18,853)	(16,007)	(12,586)
Proceeds from issuance of common stock, net	0	48,678	22
Proceeds from issuance of subordinated debt, net	0	73,516	0
Redemption of subordinated debentures, net	0	0	(8,170)
Retirement of restricted stock	(773)	(206)	(254)
Excess tax benefits	0	43	59
Exercise of stock options	321	285	124
NET CASH PROVIDED BY FINANCING ACTIVITIES	(19,305)	106,309	(20,805)
Net increase (decrease) in cash and cash equivalents	6,020	(4,246)	4,028
Cash and cash equivalents, beginning of year	11,675	15,921	11,893
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 17,695	$ 11,675	$ 15,921