Consolidated Statements Of Shareholders' Equity (USD $) In Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2010	$ 28,080	$ 956,864	$ 361,055	$ 14,680	$ (57,222)	$ 1,303,457
Net income			14,647			14,647
Other comprehensive income (loss)				2,861		2,861
Cash dividends declared, $0.34 per share			(9,175)			(9,175)
Reissuance of treasury stock under incentive plan, net of shares surrendered in payment, including tax benefit		(684)			483	(201)
Common stock issued for recognition and retention plan		(4,428)			4,428
Share-based compensation cost		2,135				2,135
Balance at Mar. 31, 2011	28,080	953,887	366,527	17,541	(52,311)	1,313,724
Balance at Dec. 31, 2011	31,163	1,135,880	375,184	24,457	(84,023)	1,482,661
Net income			19,393			19,393
Other comprehensive income (loss)				963		963
Cash dividends declared, $0.34 per share			(10,036)			(10,036)
Reissuance of treasury stock under incentive plan, net of shares surrendered in payment, including tax benefit		(489)			888	399
Common stock issued for recognition and retention plan		(6,145)			6,145
Share-based compensation cost		2,250				2,250
Balance at Mar. 31, 2012	$ 31,163	$ 1,131,496	$ 384,541	$ 25,420	$ (76,990)	$ 1,495,630