Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,232,443	$ 3,145,310	$ 3,129,276	$ 3,008,740	$ 3,069,298	$ 2,900,577	$ 3,039,068	$ 2,512,568	$ 12,515,769	$ 11,521,511	$ 10,418,178
Total benefits and expenses	2,976,949	2,805,148	2,789,961	2,800,896	2,773,755	2,612,977	2,685,845	2,404,988	11,372,954	10,477,565	9,673,383
Income before income taxes	255,494	340,162	339,315	207,844	295,543	287,600	353,223	107,580	1,142,815	1,043,946	744,795
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
Basic 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