Consolidated Statements Of Shareholders' Equity (USD $)	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2009	$ 22,107,000	$ 632,086,000	$ 341,621,000	$ 22,416,000	$ (64,015,000)	$ 954,215,000
Comprehensive income:
Net income			21,843,000			21,843,000
Change in unrealized gain on securities available for sale, net of taxes				12,165,000		12,165,000
Change in fair value of derivatives used for cash flow hedges, net of taxes				(5,422,000)		(5,422,000)
Total comprehensive income						28,586,000
Cash dividends declared, $0.68 per share			(18,221,000)			(18,221,000)
Reissuance of treasury stock under incentive compensation plans, net of shares surrendered in payment, including tax benefit		(390,000)			853,000	463,000
Common Stock Issued	5,973,000	323,007,000				328,980,000
Common stock issued for recognition and retention plan, net of shares forfeited		(5,253,000)			5,253,000
Share-based compensation cost		3,806,000				3,806,000
Balance at Jun. 30, 2010	28,080,000	953,256,000	345,243,000	29,159,000	(57,909,000)	1,297,829,000
Balance at Dec. 31, 2010	28,080,000	956,864,000	361,055,000	14,680,000	(57,222,000)	1,303,457,000
Comprehensive income:
Net income			19,833,000			19,833,000
Change in unrealized gain on securities available for sale, net of taxes				12,006,000		12,006,000
Change in fair value of derivatives used for cash flow hedges, net of taxes				(417,000)		(417,000)
Total comprehensive income						31,422,000
Cash dividends declared, $0.68 per share			(19,448,000)			(19,448,000)
Reissuance of treasury stock under incentive compensation plans, net of shares surrendered in payment, including tax benefit		(1,097,000)			6,792,000	5,695,000
Common Stock Issued	3,083,000	178,057,000				181,140,000
Common stock issued for recognition and retention plan, net of shares forfeited		(4,895,000)			4,895,000
Share-based compensation cost		4,722,000				4,722,000
Balance at Jun. 30, 2011	$ 31,163,000	$ 1,133,651,000	$ 361,440,000	$ 26,269,000	$ (45,535,000)	$ 1,506,988,000