Statements Of Changes In Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Capital In Excess Of Par Value [Member]
Balance, in shares at Dec. 31, 2012		86,165
Balance at Dec. 31, 2012	$ 1,025,969	$ 8,616	$ 882,952	$ 134,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	206,113		206,113
Balance, in shares at Dec. 31, 2013		86,165
Balance at Dec. 31, 2013	1,232,082	$ 8,616	1,089,065	134,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	267,514		267,514
Stock Repurchased During Period, Shares		(71)
Stock Repurchased and Retired During Period, Value	(5,532)	$ (7)	(5,525)
Balance, in shares at Dec. 31, 2014		86,094
Balance at Dec. 31, 2014	1,494,064	$ 8,609	1,351,054	134,401
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	304,690		304,690
Stock Repurchased During Period, Shares		(1,682)
Stock Repurchased and Retired During Period, Value	(114,117)	$ (168)	(113,949)
Balance, in shares at Dec. 31, 2015		84,412
Balance at Dec. 31, 2015	$ 1,684,637	$ 8,441	$ 1,541,795	$ 134,401