Condensed Unaudited Quarterly Financial Data (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 3,848	$ 4,665	$ 3,783	$ 3,381	$ 3,831	$ 4,490	$ 3,757	$ 3,309	$ 15,677	$ 15,387	$ 13,504
Net investment income	567	533	537	544	565	564	569	544	2,181	2,242	2,070
Net realized gains (losses) including OTTI	272	(60)	(394)	260	83	(760)	(73)	(45)	78	(795)	432
Total revenues	4,687	5,138	3,926	4,185	4,479	4,294	4,253	3,808	17,936	16,834	16,006
Losses and loss expenses	2,683	3,047	2,119	1,804	2,286	2,745	2,226	2,263	9,653	9,520	7,579
Policy benefits	142	130	102	147	119	83	108	91	521	401	357
Net income (loss)	$ 765	$ 640	$ 328	$ 973	$ 735	$ (39)	$ 594	$ 250	$ 2,706	$ 1,540	$ 3,085
Basic earnings per share (US$ per share)	$ 2.24	$ 1.88	$ 0.96	$ 2.87	$ 2.17	$ (0.11)	$ 1.75	$ 0.74	$ 7.96	$ 4.55	$ 9.08
Diluted earnings per share (US$ per share)	$ 2.22	$ 1.86	$ 0.96	$ 2.84	$ 2.15	$ (0.11)	$ 1.74	$ 0.73	$ 7.89	$ 4.52	$ 9.04