Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,069,298	$ 2,900,577	$ 3,039,068	$ 2,512,568	$ 2,828,591	$ 2,438,634	$ 2,630,340	$ 2,520,613	$ 11,521,511	$ 10,418,178	$ 10,904,194
Total benefits and expenses	2,773,755	2,612,977	2,685,845	2,404,988	2,621,848	2,298,497	2,416,550	2,336,488	10,477,565	9,673,383	9,895,661
Income before income taxes	295,543	287,600	353,223	107,580	206,743	140,137	213,790	184,125	1,043,946	744,795	1,008,533
Net Income (Loss) Attributable to Parent	$ 190,149	$ 198,719	$ 236,103	$ 76,472	$ 163,127	$ 83,534	$ 130,391	$ 125,114	$ 701,443	$ 502,166	$ 684,047
Basic earnings per share:
Earnings Per Share, Basic (in dollars per share)	$ 2.96	$ 3.10	$ 3.68	$ 1.18	$ 2.49	$ 1.26	$ 1.97	$ 1.84	$ 10.91	$ 7.55	$ 9.88
Earnings Per Share, Diluted (in dollars per share)	$ 2.92	$ 3.07	$ 3.64	$ 1.17	$ 2.46	$ 1.25	$ 1.94	$ 1.81	$ 10.79	$ 7.46	$ 9.78