Earnings Per Share (Reconciliation of the Numerators and Denominators of the Basic and Diluted Per Shares) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 232,190	$ 236,103	$ 377,702	$ 312,575
Shares:
Weighted average outstanding shares (denominator for basic calculation)	64,449	64,126	64,401	64,348
Equivalent shares from outstanding stock options	1,159	670	1,204	660
Denominator for diluted calculation	65,608	64,796	65,605	65,008
Earnings Per Share:
Basic earnings per share	$ 3.60	$ 3.68	$ 5.86	$ 4.86
Diluted earnings per share	$ 3.54	$ 3.64	$ 5.76	$ 4.81