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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to Parent	$ 110,039	$ 301,199	$ 204,374	$ 100,230	$ 1,216,888	$ 227,591	$ 232,190	$ 145,512	$ 715,842	$ 1,822,181	$ 701,443
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted Average Number of Shares Outstanding, Basic									63,658	64,427	64,274
Equivalent shares from outstanding stock options									1,436	1,326	715
Weighted Average Number of Shares Outstanding, Diluted									65,094	65,753	64,989
Earnings per share
Earnings Per Share, Basic (in dollars per share)	$ 1.75	$ 4.76	$ 3.19	$ 1.55	$ 18.89	$ 3.53	$ 3.60	$ 2.26	$ 11.25	$ 28.28	$ 10.91
Earnings Per Share, Diluted (in dollars per share)	$ 1.72	$ 4.68	$ 3.13	$ 1.52	$ 18.49	$ 3.47	$ 3.54	$ 2.22	$ 11.00	$ 27.71	$ 10.79