Reinsurance (Consolidated Reinsurance) (Details) (USD $) In Millions, 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
Premiums written [Abstract]
Direct									$ 18,144	$ 17,626	$ 15,887
Assumed									3,449	3,205	3,624
Ceded									(5,518)	(5,459)	(5,803)
Net									16,075	15,372	13,708
Premiums earned [Abstract]
Direct									17,802	17,534	15,780
Assumed									3,302	3,349	3,516
Ceded									(5,427)	(5,496)	(5,792)
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