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)
Treasury stock, Beginning Balance (in shares) at Dec. 31, 2011			(410,403)
Common stock issued, Beginning Balance (in shares) at Dec. 31, 2011		60,702,342
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans, Value	5,776	255	136	5,385
Common stock issued for compensatory plans (in shares)		254,540	4,002
Treasury Stock Acquired [Abstract]
Repurchase of common stock, Value	(8,006)		(8,006)
Repurchase of common stock (in shares)			(200,000)
Dividends on common stock	(58,673)				(58,673)
Net income	140,536				140,536
Other comprehensive income, net of tax	2,364					2,364
Stockholders' equity, Ending Balance at Sep. 30, 2012	1,501,854	60,957	(21,085)	415,289	1,072,468	(25,775)
Treasury stock, Ending Balance (in shares) at Sep. 30, 2012			(606,401)
Common stock issued, Ending Balance (in shares) at Sep. 30, 2012		60,956,882
Stockholders' equity, Beginning Balance at Dec. 31, 2012	1,499,213	60,962	(21,072)	416,619	1,075,074	(32,370)
Treasury stock, Beginning Balance (in shares) at Dec. 31, 2012	(606,025)		(606,025)
Common stock issued, Beginning Balance (in shares) at Dec. 31, 2012	60,961,570	60,961,570
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans, Value	4,768	85	389	4,294
Common stock issued for compensatory plans (in shares)		85,436	11,177
Treasury Stock Acquired [Abstract]
Dividends on common stock	(64,684)				(64,684)
Net income	135,572				135,572
Other comprehensive income, net of tax	3,045					3,045
Stockholders' equity, Ending Balance at Sep. 30, 2013	$ 1,577,914	$ 61,047	$ (20,683)	$ 420,913	$ 1,145,962	$ (29,325)
Treasury stock, Ending Balance (in shares) at Sep. 30, 2013	(594,848)		(594,848)
Common stock issued, Ending Balance (in shares) at Sep. 30, 2013	61,047,006	61,047,006