Condensed Financial Information - Parent Company - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Interest-bearing deposits in subsidiary bank	$ 40,812	$ 162,764
Total cash and cash equivalents	248,647	372,347	$ 256,486	$ 272,886
OTHER ASSETS	90,762	75,725
Total assets	7,731,854	7,254,715
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	6,678,559	6,331,947
SHAREHOLDERS' EQUITY:
Common stock	678	663
Capital surplus	443,114	378,062
Retained earnings	606,658	517,257
Accumulated other comprehensive earnings	2,845	26,786
Total shareholders' equity	1,053,295	922,768	837,885	804,986
Total liabilities and shareholders' equity	7,731,854	7,254,715
Subsidiary Bank [Member]
ASSETS
Cash in banks	16,981	14,272
Interest-bearing deposits in subsidiary bank	84,279	64,195
Unaffiliated Banks [Member]
ASSETS
Cash in banks	2	2
Parent Company [Member]
ASSETS
Total cash and cash equivalents	101,262	78,469	$ 93,251	$ 90,473
SECURITIES AVAILABLE-FOR-SALE, at fair value	6,276	8,515
Investment in and advances to subsidiaries, at equity	959,352	847,445
INTANGIBLE ASSETS	723	723
OTHER ASSETS	2,647	2,654
Total assets	1,070,260	937,806
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	16,965	15,038
SHAREHOLDERS' EQUITY:
Common stock	678	663
Capital surplus	443,114	378,062
Retained earnings	606,658	517,257
Treasury stock	(7,507)	(7,148)
Deferred compensation	7,507	7,148
Accumulated other comprehensive earnings	2,845	26,786
Total shareholders' equity	1,053,295	922,768
Total liabilities and shareholders' equity	$ 1,070,260	$ 937,806