Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Hardy segment) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (81)	$ (40)	$ (112)	$ (110)
Hardy [Member]
Marine and Aviation		1	3	1
Non-Marine Property	1	(5)	8	(5)
Property Treaty	(5)		(3)
Specialty	(2)	(2)	(3)	(2)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (6)	$ (6)	$ 5	$ (6)