Condensed Financial Information - Parent Company Only - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	$ 63,401	$ 52,580	$ 41,518
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Change in market value of equity securities	583	0	0
Excess tax benefits	318	587	0
Increase in other assets	2,679	(25,065)	(5,600)
(Decrease) increase in other liabilities	9,743	3,705	1,618
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of available for sale securities	(110,370)	(140,258)	(244,861)
Purchase of equity securities	(570)	(307)	(838)
Proceeds from sale of available for sale securities	0	4,500	15,654
Proceeds from sales of equity securities	2,155	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(21,307)	(18,853)	(16,007)
Proceeds from issuance of common stock, net	0	0	48,678
Proceeds from issuance of subordinated debt, net	0	0	73,516
Retirement of restricted stock	(763)	(773)	(206)
Excess tax benefits	0	0	43
Exercise of stock options	307	321	285
Net increase (decrease) in cash and cash equivalents	65,666	(32,868)	57,308
Cash and cash equivalents, beginning of year	142,933	175,801	118,493
CASH AND CASH EQUIVALENTS, END OF YEAR	208,599	142,933	175,801
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	63,401	52,580	41,518
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Gain on securities	0	(2,539)	0
Amortization of subordinated debt costs	125	118	30
Change in market value of equity securities	338	0	0
Excess tax benefits	318	587	0
Increase in other assets	(1,446)	(1,927)	(922)
(Decrease) increase in other liabilities	(6)	(17)	1,010
Equity in undistributed income of subsidiaries	(37,454)	(26,615)	(22,400)
NET CASH PROVIDED BY OPERATING ACTIVITIES	25,276	22,187	19,236
CASH FLOWS FROM INVESTING ACTIVITIES
Net cash used in acquisition	0	0	(5,356)
Purchases of available for sale securities	0	(79)	(62)
Purchase of equity securities	(78)	0	0
Proceeds from sale of available for sale securities	0	3,217	0
Proceeds from sales of equity securities	2,155	0	0
Contribution to subsidiary	0	0	(124,373)
NET CASH USED IN INVESTING ACTIVITIES	2,077	3,138	(129,791)
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(21,307)	(18,853)	(16,007)
Proceeds from issuance of common stock, net	0	0	48,678
Proceeds from issuance of subordinated debt, net	0	0	73,516
Retirement of restricted stock	(763)	(773)	(206)
Excess tax benefits	0	0	43
Exercise of stock options	307	321	285
NET CASH PROVIDED BY FINANCING ACTIVITIES	(21,763)	(19,305)	106,309
Net increase (decrease) in cash and cash equivalents	5,590	6,020	(4,246)
Cash and cash equivalents, beginning of year	17,695	11,675	15,921
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 23,285	$ 17,695	$ 11,675