Consolidated Statement of Changes in Common Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Treasury Stock [Member]	Premium on Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2012	$ 1,499,213	$ 60,962	$ (21,072)	$ 416,619	$ 1,075,074	$ (32,370)
Balance (in shares) at Dec. 31, 2012			(606,025)
Balance (in shares) at Dec. 31, 2012		60,961,570
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans	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
Balances at Sep. 30, 2013	1,577,914	61,047	(20,683)	420,913	1,145,962	(29,325)
Balance (in shares) at Sep. 30, 2013			(594,848)
Balance (in shares) at Sep. 30, 2013		61,047,006
Balances at Dec. 31, 2013	1,586,197	61,047	(20,601)	422,624	1,149,003	(25,876)
Balance (in shares) at Dec. 31, 2013	(592,486)		(592,486)
Balance (in shares) at Dec. 31, 2013	61,047,006	61,047,006
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans	716	4	5,980	(5,268)
Common stock issued for compensatory plans (in shares)		4,280	168,866
Treasury Stock Acquired [Abstract]
Repurchase of common stock	(12,449)		(12,449)
Repurchase of common stock (in shares)			(250,000)
Dividends on common stock	(70,744)				(70,744)
Net income	133,392				133,392
Other comprehensive income, net of tax	1,759					1,759
Balances at Sep. 30, 2014	$ 1,638,871	$ 61,051	$ (27,070)	$ 417,356	$ 1,211,651	$ (24,117)
Balance (in shares) at Sep. 30, 2014	(673,620)		(673,620)
Balance (in shares) at Sep. 30, 2014	61,051,286	61,051,286