Condensed Financial Information - Parent Company - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Interest-bearing deposits in subsidiary bank	$ 48,574	$ 89,936
Total cash and cash equivalents	256,486	272,886	$ 253,471	$ 212,012
OTHER ASSETS	86,066	85,635
Total assets	6,809,931	6,665,070
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	5,972,046	5,860,084
SHAREHOLDERS' EQUITY:
Common stock	661	660
Capital surplus	372,245	368,925
Retained earnings	446,534	388,006
Treasury stock	(6,671)	(6,296)
Deferred Compensation	6,671	6,296
Accumulated other comprehensive earnings	18,445	47,395
Total shareholders' equity	837,885	804,986	681,537	587,647
Total liabilities and shareholders' equity	6,809,931	6,665,070
Subsidiary Bank [Member]
ASSETS
Cash in banks	15,070	12,311
Interest-bearing deposits in subsidiary bank	78,179	78,160
Unaffiliated Banks [Member]
ASSETS
Cash in banks	2	2
Parent Company [Member]
ASSETS
Total cash and cash equivalents	93,251	90,473	$ 70,085	$ 76,070
SECURITIES AVAILABLE-FOR-SALE, at fair value	11,593	12,062
Investment in and advances to subsidiaries, at equity	744,971	713,909
INTANGIBLE ASSETS	723	723
OTHER ASSETS	2,668	3,693
Total assets	853,206	820,860
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	15,321	15,874
SHAREHOLDERS' EQUITY:
Common stock	661	660
Capital surplus	372,245	368,925
Retained earnings	446,534	388,006
Treasury stock	(6,671)	(6,296)
Deferred Compensation	6,671	6,296
Accumulated other comprehensive earnings	18,445	47,395
Total shareholders' equity	837,885	804,986
Total liabilities and shareholders' equity	$ 853,206	$ 820,860