Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,278,518	$ 3,227,531	$ 3,195,908	$ 3,173,707	$ 3,232,443	$ 3,145,310	$ 3,129,276	$ 3,008,740	$ 12,875,664	$ 12,515,769	$ 11,521,511
Total benefits and expenses	3,140,572	2,904,870	2,948,620	3,035,782	2,976,949	2,805,148	2,789,961	2,800,896	12,029,844	11,372,954	10,477,565
Income before income taxes	137,946	322,661	247,288	137,925	255,494	340,162	339,315	207,844	845,820	1,142,815	1,043,946
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
Basic 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