Condensed Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 4,207	$ 4,719	$ 4,360	$ 3,927		$ 4,370	$ 4,754	$ 4,332	$ 3,970	$ 17,213	$ 17,426	$ 16,613
Net investment income	532	549	562	551		577	566	556	553	2,194	2,252	2,144
Net realized gains (losses) including OTTI	(60)	(397)	126	(89)		(210)	(120)	(73)	(104)	(420)	(507)	504
Total revenues	4,679	4,871	5,048	4,389		4,737	5,200	4,815	4,419	18,987	19,171	19,261
Losses and loss expenses	2,302	2,643	2,417	2,122		2,416	2,684	2,388	2,161	9,484	9,649	9,348
Policy benefits	159	89	153	142		134	125	144	114	543	517	515
Net income	$ 683	$ 528	$ 942	$ 681	[1]	$ 555	$ 785	$ 779	$ 734	$ 2,834	$ 2,853	$ 3,758
Basic earnings per share	$ 2.10	$ 1.63	$ 2.89	$ 2.08		$ 1.68	$ 2.35	$ 2.30	$ 2.16	$ 8.71	$ 8.50	$ 11.02
Diluted earnings per share	$ 2.08	$ 1.62	$ 2.86	$ 2.05		$ 1.66	$ 2.32	$ 2.28	$ 2.14	$ 8.62	$ 8.42	$ 10.92
Income Tax Reconciliation, Change To Deferred Taxes Related to Unrealized Foreign Exchange Net	$ 89

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