Condensed Financial Information - Parent Company - Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ASSETS
INTEREST-BEARING DEPOSITS IN BANKS	$ 139,676,000	$ 104,597,000
Total cash and cash equivalents	360,739,000	250,836,000	264,267,000	264,918,000
Interest-bearing time deposits in unaffiliated banks	49,005,000	61,175,000
Securities available-for-sale, at fair value	1,819,035,000	1,841,389,000
Intangible assets	71,973,000	72,122,000
Other assets	62,293,000	62,688,000
Total assets	4,502,012,000	4,120,531,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	3,945,049,000	3,611,994,000
SHAREHOLDERS' EQUITY:
Common stock	315,000	314,000
Capital surplus	277,412,000	276,127,000
Retained earnings	227,927,000	184,871,000
Treasury shares	5,007,000	4,597,000
Deferred Compensation	5,007,000	4,597,000
Accumulated other comprehensive earnings	51,309,000	47,225,000
Total shareholders' equity	556,963,000	508,537,000	441,688,000	415,702,000
Total liabilities and shareholders' equity	4,502,012,000	4,120,531,000
Subsidiary bank [Member]
ASSETS
Cash in banks	4,780,000	16,800,000
INTEREST-BEARING DEPOSITS IN BANKS	67,263,000	28,192,000
Unaffiliated banks [Member]
ASSETS
Cash in banks	2,000	2,000
INTEREST-BEARING DEPOSITS IN BANKS		5,026,000
Interest-bearing time deposits in unaffiliated banks	3,078,000	2,118,000
Parent company [Member]
ASSETS
Total cash and cash equivalents	72,045,000	50,020,000	31,905,000	41,605,000
Securities available-for-sale, at fair value	12,706,000	17,729,000
Investment in and advances to subsidiaries, at equity	468,502,000	446,320,000
Intangible assets	723,000	723,000
Other assets	1,961,000	2,009,000
Total assets	559,015,000	518,919,000
LIABILITIES AND SHAREHOLDERS' EQUITY
Total liabilities	2,052,000	10,382,000
SHAREHOLDERS' EQUITY:
Common stock	315,000	315,000
Capital surplus	277,412,000	276,126,000
Retained earnings	227,927,000	184,871,000
Treasury shares	(5,007,000)	(4,597,000)
Deferred Compensation	5,007,000	4,597,000
Accumulated other comprehensive earnings	51,309,000	47,225,000
Total shareholders' equity	556,963,000	508,537,000
Total liabilities and shareholders' equity	$ 559,015,000	$ 518,919,000