Consolidated Statements of Changes in Common Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Premium on Common Stock [Member]	Retained Earnings [Member]	AOCI [Member]
Common stock issued, Beginning Balance (in shares) at Dec. 31, 2012		60,961,570
Treasury stock, Beginning Balance (in shares) at Dec. 31, 2012			(606,025)
Balances. Beginning at Dec. 31, 2012	$ 1,499,213	$ 60,962	$ (21,072)	$ 416,619	$ 1,075,074	$ (32,370)
Stock Issued During Period, Value, Share-based Compensation [Abstract]
Common stock issued for compensatory plans (in shares)		85,436	13,539
Treasury Stock Acquired [Abstract]
Common stock issued for compensatory plans	6,561	$ 85	$ 471	6,005
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
Common stock issued, Ending Balance (in shares) at Dec. 31, 2013		61,047,006
Treasury stock, Ending Balance (in shares) at Dec. 31, 2013			(592,486)
Balances, Ending at Dec. 31, 2013	1,586,197	$ 61,047	$ (20,601)	422,624	1,149,003	(25,876)
Stock Issued During Period, Value, Share-based Compensation [Abstract]
Common stock issued for compensatory plans (in shares)		4,280	212,667
Treasury Stock Acquired [Abstract]
Repurchase of common stock (in shares)			(250,000)
Repurchase of common stock	(12,449)		$ (12,449)
Common stock issued for compensatory plans	602	$ 4	$ 7,740	(7,142)
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)
Common stock issued, Ending Balance (in shares) at Dec. 31, 2014	61,051,286	61,051,286
Treasury stock, Ending Balance (in shares) at Dec. 31, 2014	(629,819)		(629,819)
Balances, Ending at Dec. 31, 2014	$ 1,627,270	$ 61,051	$ (25,310)	415,482	1,208,712	(32,665)
Stock Issued During Period, Value, Share-based Compensation [Abstract]
Common stock issued for compensatory plans (in shares)		7,632	53,369
Treasury Stock Acquired [Abstract]
Common stock issued for compensatory plans	5,189	$ 8	$ 2,145	3,036
Dividends on common stock	(97,367)				(97,367)
Net income	133,669				133,669
Other comprehensive income (loss), net of tax	$ 6,080					6,080
Common stock issued, Ending Balance (in shares) at Dec. 31, 2015	61,058,918	61,058,918
Treasury stock, Ending Balance (in shares) at Dec. 31, 2015	(576,450)		(576,450)
Balances, Ending at Dec. 31, 2015	$ 1,674,841	$ 61,059	$ (23,165)	$ 418,518	$ 1,245,014	$ (26,585)