Reinsurance (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Effect of reinsurance on the premiums written and earned [Abstract]
Direct premiums written	$ 12,302	$ 11,952	$ 11,813
Reinsurance assumed	548	391	370
Reinsurance ceded	(1,092)	(1,107)	(1,106)
Net premiums written	11,758	11,236	11,077
Direct premiums earned	12,247	11,949	12,058
Reinsurance assumed	505	363	435
Reinsurance ceded	(1,108)	(1,097)	(1,162)
Net premiums earned	$ 11,644	$ 11,215	$ 11,331