Condensed Unaudited Quarterly Financial Data (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 4,363	$ 4,610	$ 4,067	$ 3,573	$ 3,848	$ 4,665	$ 3,783	$ 3,381	$ 16,613	$ 15,677	$ 15,387
Net investment income	557	522	534	531	567	533	537	544	2,144	2,181	2,242
Net realized gains (losses) including OTTI	154	40	104	206	272	(60)	(394)	260	504	78	(795)
Total revenues	5,074	5,172	4,705	4,310	4,687	5,138	3,926	4,185	19,261	17,936	16,834
Losses and loss expenses	2,517	2,655	2,250	1,926	2,683	3,047	2,119	1,804	9,348	9,653	9,520
Policy benefits	136	138	110	131	142	130	102	147	515	521	401
Net income	$ 998	$ 916	$ 891	$ 953	$ 765	$ 640	$ 328	$ 973	$ 3,758	$ 2,706	$ 1,540
Basic earnings per share	$ 2.93	$ 2.68	$ 2.61	$ 2.80	$ 2.24	$ 1.88	$ 0.96	$ 2.87	$ 11.02	$ 7.96	$ 4.55
Diluted earnings per share	$ 2.90	$ 2.66	$ 2.59	$ 2.77	$ 2.22	$ 1.86	$ 0.96	$ 2.84	$ 10.92	$ 7.89	$ 4.52