Consolidated Statements of Changes in Common Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Premium on Common Stock [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]
Stockholders' Equity, Beginning Balance at Dec. 31, 2011	$ 1,419,857	$ 60,702	$ 409,904	$ 990,605	$ (13,215)	$ (28,139)
Treasury stock, Beginning Balance (shares) at Dec. 31, 2011					(410,403)
Common stock issued, Beginning Balance (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	7,125	260	6,715		150
Common stock issued for compensatory plans (in shares)		259,228			4,378
Treasury Stock Acquired [Abstract]
Repurchase of common stock, Value	(8,007)				(8,007)
Repurchase of common stock (in shares)					(200,000)
Dividends on common stock	(79,179)			(79,179)
Net income	163,648			163,648
Other comprehensive income (loss), net of tax	(4,231)					(4,231)
Stockholders' Equity, Ending Balance at Dec. 31, 2012	1,499,213	60,962	416,619	1,075,074	(21,072)	(32,370)
Treasury stock, Ending Balance (shares) at Dec. 31, 2012					(606,025)
Common stock issued, Ending Balance (shares) at Dec. 31, 2012		60,961,570
Stock Issued or Granted During Period, Share-based Compensation [Abstract]
Common stock issued for compensatory plans, Value	6,561	85	6,005		471
Common stock issued for compensatory plans (in shares)		85,436			13,539
Treasury Stock Acquired [Abstract]
Dividends on common stock	(86,756)			(86,756)
Net income	160,685			160,685
Other comprehensive income (loss), net of tax	6,494					6,494
Stockholders' Equity, Ending Balance at Dec. 31, 2013	1,586,197	61,047	422,624	1,149,003	(20,601)	(25,876)
Treasury stock, Ending Balance (shares) at Dec. 31, 2013	(592,486)				(592,486)
Common stock issued, Ending Balance (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, Value	602	4	(7,142)		7,740
Common stock issued for compensatory plans (in shares)		4,280			212,667
Treasury Stock Acquired [Abstract]
Repurchase of common stock, Value	(12,449)				(12,449)
Repurchase of common stock (in shares)					(250,000)
Dividends on common stock	(95,030)			(95,030)
Net income	154,739			154,739
Other comprehensive income (loss), net of tax	(6,789)					(6,789)
Stockholders' Equity, Ending Balance at Dec. 31, 2014	$ 1,627,270	$ 61,051	$ 415,482	$ 1,208,712	$ (25,310)	$ (32,665)
Treasury stock, Ending Balance (shares) at Dec. 31, 2014	(629,819)				(629,819)
Common stock issued, Ending Balance (shares) at Dec. 31, 2014	61,051,286	61,051,286