Earnings Per Share (Schedule of Earnings Per Share Basic and Diluted) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to Parent	$ 1,216,888	$ 227,591	$ 232,190	$ 145,512	$ 190,149	$ 198,719	$ 236,103	$ 76,472	$ 1,822,181	$ 701,443	$ 502,166
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted Average Number of Shares Outstanding, Basic									64,427	64,274	66,553
Equivalent shares from outstanding stock options									1,326	715	739
Weighted Average Number of Shares Outstanding, Diluted									65,753	64,989	67,292
Earnings per share
Earnings Per Share, Basic (in dollars per share)	$ 18.89	$ 3.53	$ 3.60	$ 2.26	$ 2.96	$ 3.10	$ 3.68	$ 1.18	$ 28.28	$ 10.91	$ 7.55
Earnings Per Share, Diluted (in dollars per share)	$ 18.49	$ 3.47	$ 3.54	$ 2.22	$ 2.92	$ 3.07	$ 3.64	$ 1.17	$ 27.71	$ 10.79	$ 7.46