Consolidated Statement of Changes in Common Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders' Equity, Beginning Balance at Dec. 31, 2011	$ 1,419,857	$ 60,702	$ (13,215)	$ 409,904	$ 990,605	$ (28,139)
Common Stock, Shares, Issued, Beginning Balance (in shares) at Dec. 31, 2011		60,702,342
Treasury Stock, Shares, Outstanding, Beginning Balance (in shares) at Dec. 31, 2011			(410,403)
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans, Value	4,313	247	122	3,944	0
Common stock issued for compensatory plans, Shares		246,738	3,561
Treasury Stock Acquired [Abstract]
Repurchase of common stock, Value	(8,007)		(8,007)
Repurchase of common stock, Shares			(200,000)
Dividends on common stock	(38,163)				(38,163)
Net income	76,718				76,718
Other comprehensive income, net of tax	1,549					1,549
Stockholders' Equity, Ending Balance at Jun. 30, 2012	1,456,267	60,949	(21,100)	413,848	1,029,160	(26,590)
Common Stock, Shares, Issued, Ending Balance (in shares) at Jun. 30, 2012		60,949,080
Treasury Stock, Shares, Outstanding, Ending Balance (in shares) at Jun. 30, 2012			(606,842)
Stockholders' Equity, Beginning Balance at Dec. 31, 2012	1,499,213	60,962	(21,072)	416,619	1,075,074	(32,370)
Common Stock, Shares, Issued, Beginning Balance (in shares) at Dec. 31, 2012	60,961,570	60,961,570
Treasury Stock, Shares, Outstanding, Beginning Balance (in shares) at Dec. 31, 2012			(606,025)
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans, Value	3,074	85	286	2,703	0
Common stock issued for compensatory plans, Shares		85,436	8,233
Treasury Stock Acquired [Abstract]
Dividends on common stock	(42,614)				(42,614)
Net income	69,166				69,166
Other comprehensive income, net of tax	2,495					2,495
Stockholders' Equity, Ending Balance at Jun. 30, 2013	$ 1,531,334	$ 61,047	$ (20,786)	$ 419,322	$ 1,101,626	$ (29,875)
Common Stock, Shares, Issued, Ending Balance (in shares) at Jun. 30, 2013	61,047,006	61,047,006
Treasury Stock, Shares, Outstanding, Ending Balance (in shares) at Jun. 30, 2013			(597,792)