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
Quarterly Financial Data [Abstract]
Revenues	$ 3,345	$ 3,420	$ 3,400	$ 3,420	$ 3,411	$ 3,267	$ 3,318	$ 3,323	$ 13,585	$ 13,319	$ 13,016
Losses and expenses	2,743	3,054	2,854	2,735	2,557	2,483	2,616	2,675	11,386	10,331	10,054
Federal and Foreign Income Tax	150	68	127	176	234	212	184	184	521	814	779
NET INCOME	$ 452	$ 298	$ 419	$ 509	$ 620	$ 572	$ 518	$ 464	$ 1,678	$ 2,174	$ 2,183
Basic earnings per share	$ 1.62	$ 1.04	$ 1.43	$ 1.71	$ 2.03	$ 1.82	$ 1.60	$ 1.39	$ 5.80	$ 6.81	$ 6.24
Diluted earnings per share	$ 1.6	$ 1.04	$ 1.42	$ 1.70	$ 2.02	$ 1.80	$ 1.59	$ 1.39	$ 5.76	$ 6.76	$ 6.18
Underwriting ratios [Abstract]
Losses to premiums earned	0.593	0.702	0.636	0.620	0.561	0.545	0.595	0.623
Expenses to premiums written	0.306	0.324	0.313	0.317	0.309	0.317	0.309	0.313
Combined loss and expense ratio	0.899	1.026	0.949	0.937	0.870	0.862	0.904	0.936