Condensed Financial Information - Parent Company - Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
ASSETS
Interest-bearing deposits in subsidiary bank	$ 54,324,000	$ 25,498,000
Total cash and cash equivalents	253,471,000	212,012,000	360,739,000	250,836,000
Interest-bearing time deposits in unaffiliated banks	17,002,000	31,917,000
SECURITIES AVAILABLE-FOR-SALE, at fair value	2,415,856,000	2,057,723,000
INTANGIBLE ASSETS	97,359,000	97,485,000
OTHER ASSETS	59,906,000	71,334,000
Total assets	5,848,202,000	5,222,208,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	5,166,665,000	4,634,561,000
SHAREHOLDERS' EQUITY:
Common stock	641,000	320,000
Capital surplus	305,429,000	302,991,000
Retained earnings	327,978,000	273,972,000
Treasury shares	(5,878,000)	(5,490,000)
Deferred Compensation	5,878,000	5,490,000
Accumulated other comprehensive earnings	47,489,000	10,364,000
Total shareholders' equity	681,537,000	587,647,000	556,963,000	508,537,000
Total liabilities and shareholders' equity	5,848,202,000	5,222,208,000
Subsidiary Bank [Member]
ASSETS
Cash in banks	9,441,000	11,960,000
Interest-bearing deposits in subsidiary bank	60,642,000	64,108,000
Unaffiliated Banks [Member]
ASSETS
Cash in banks	2,000	2,000
Interest-bearing time deposits in unaffiliated banks		480,000
Parent Company [Member]
ASSETS
Total cash and cash equivalents	70,085,000	76,070,000	72,045,000	50,020,000
SECURITIES AVAILABLE-FOR-SALE, at fair value	12,243,000	11,701,000
Investment in and advances to subsidiaries, at equity	606,415,000	507,074,000
INTANGIBLE ASSETS	723,000	723,000
OTHER ASSETS	2,787,000	2,054,000
Total assets	692,253,000	598,102,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	10,716,000	10,455,000
SHAREHOLDERS' EQUITY:
Common stock	641,000	320,000
Capital surplus	305,429,000	302,991,000
Retained earnings	327,978,000	273,972,000
Treasury shares	(5,878,000)	(5,490,000)
Deferred Compensation	5,878,000	5,490,000
Accumulated other comprehensive earnings	47,489,000	10,364,000
Total shareholders' equity	681,537,000	587,647,000
Total liabilities and shareholders' equity	$ 692,253,000	$ 598,102,000