Condensed Unaudited Quarterly Financial Data (Schedule Of Condensed Unaudited Quarterly Financial Data) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Unaudited Quarterly Financial Data [Abstract]
Net premiums earned	$ 3,831	$ 4,490	$ 3,757	$ 3,309	$ 3,572	$ 3,422	$ 3,233	$ 3,277	$ 15,387	$ 13,504	$ 13,240
Net investment income	565	564	569	544	532	516	518	504	2,242	2,070	2,031
Net realized gains (losses) including OTTI	83	(760)	(73)	(45)	305	(50)	9	168	(795)	432	(196)
Total revenues	4,479	4,294	4,253	3,808	4,409	3,888	3,760	3,949	16,834	16,006	15,075
Losses and loss expenses	2,286	2,745	2,226	2,263	1,971	1,887	1,800	1,921	9,520	7,579	7,422
Policy benefits	119	83	108	91	90	93	87	87	401	357	325
Net income	$ 750	$ (31)	$ 607	$ 259	$ 1,001	$ 675	$ 677	$ 755	$ 1,585	$ 3,108	$ 2,549
Basic earnings per share	$ 2.22	$ (0.09)	$ 1.79	$ 0.77	$ 2.94	$ 1.98	$ 1.99	$ 2.23	$ 4.68	$ 9.15	$ 7.57
Diluted earnings per share	$ 2.20	$ (0.09)	$ 1.77	$ 0.76	$ 2.92	$ 1.97	$ 1.98	$ 2.22	$ 4.65	$ 9.11	$ 7.55