Condensed Financial Information - Parent Company Only - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	$ 70,672	$ 63,401	$ 52,580
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Change in market value of equity securities	(496)	583	0
Excess tax benefits	189	318	587
Increase in other assets	(15,986)	2,679	(25,065)
Increase (decrease) in other liabilities	15,092	9,743	3,705
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of equity securities	(1,343)	(570)	(307)
Proceeds from sales of equity securities	1,287	2,155	0
Net cash received from business acquisition	13,454	0	0
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(24,919)	(21,307)	(18,853)
Retirement of restricted stock	(715)	(763)	(773)
Exercise of stock options	313	307	321
Cash and cash equivalents, beginning of year	208,599	142,933	175,801
CASH AND CASH EQUIVALENTS, END OF YEAR	282,371	208,599	142,933
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES
NET INCOME	70,672	63,401	52,580
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Gain on securities	0	0	(2,539)
Amortization of subordinated debt costs	36	125	118
Change in market value of equity securities	(197)	338	0
Excess tax benefits	189	318	587
Increase in other assets	(1,873)	(1,446)	(1,927)
Increase (decrease) in other liabilities	121	(6)	(17)
Equity in undistributed income of subsidiaries	(38,160)	(37,454)	(26,615)
NET CASH PROVIDED BY OPERATING ACTIVITIES	30,788	25,276	22,187
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of available for sale securities			(79)
Purchase of equity securities	(82)	(78)	0
Proceeds from sale of available for sale securities			3,217
Proceeds from sales of equity securities	1,287	2,155	0
Net cash received from business acquisition	24	0	0
NET CASH PROVIDED BY INVESTING ACTIVITIES	1,229	2,077	3,138
CASH FLOWS FROM FINANCING ACTIVITIES
Cash dividends paid on common stock	(24,919)	(21,307)	(18,853)
Retirement of restricted stock	(715)	(763)	(773)
Exercise of stock options	313	307	321
NET CASH USED IN FINANCING ACTIVITIES	(25,321)	(21,763)	(19,305)
Net increase in cash and cash equivalents	6,696	5,590	6,020
Cash and cash equivalents, beginning of year	23,285	17,695	11,675
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 29,981	$ 23,285	$ 17,695