Condensed Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	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
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 4,207	$ 4,719	$ 4,360	$ 3,927	$ 28,749	$ 17,213	$ 17,426
Net Investment Income	744	739	708	674	532	549	562	551	2,865	2,194	2,252
Net realized gains (losses) including OTTI	365	100	(216)	(394)	(60)	(397)	126	(89)	(145)	(420)	(507)
Total revenues	8,168	8,527	7,897	6,877	4,679	4,871	5,048	4,389	31,469	18,987	19,171
Losses and loss expenses	3,855	4,269	4,254	3,674	2,302	2,643	2,417	2,122	16,052	9,484	9,649
Policy benefits	161	155	146	126	159	89	153	142	588	543	517
Net income	$ 1,610	$ 1,360	$ 726	$ 439	$ 683	$ 528	$ 942	$ 681	$ 4,135	$ 2,834	$ 2,853
Basic earnings per share	$ 3.44	$ 2.90	$ 1.55	$ 0.98	$ 2.10	$ 1.63	$ 2.89	$ 2.08	$ 8.94	$ 8.71	$ 8.50
Diluted earnings per share	$ 3.41	$ 2.88	$ 1.54	$ 0.97	$ 2.08	$ 1.62	$ 2.86	$ 2.05	$ 8.87	$ 8.62	$ 8.42