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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income (numerator for basic and diluted calculations)	$ 227,591	$ 198,719	$ 605,293	$ 511,294
Shares:
Weighted average outstanding shares (denominator for basic calculation)	64,488	64,146	64,430	64,281
Equivalent shares from outstanding stock options	1,165	669	1,174	663
Denominator for diluted calculation	65,653	64,815	65,604	64,944
Earnings Per Share:
Basic earnings per share	$ 3.53	$ 3.10	$ 9.39	$ 7.95
Diluted earnings per share	$ 3.47	$ 3.07	$ 9.23	$ 7.87