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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 198,719	$ 83,534	$ 511,294	$ 339,039
Shares:
Weighted average outstanding shares (denominator for basic calculation)	64,146	66,205	64,281	66,895
Equivalent shares from outstanding stock options	669	677	663	749
Denominator for diluted calculation	64,815	66,882	64,944	67,644
Earnings Per Share:
Basic earnings per share	$ 3.10	$ 1.26	$ 7.95	$ 5.07
Diluted earnings per share	$ 3.07	$ 1.25	$ 7.87	$ 5.01