Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (210)	$ (21)	$ (118)
Specialty [Member]
Medical Professional Liability	(43)	39	(27)
Other Professional Liability and Management Liability	0	(87)	(73)
Surety	(69)	(82)	(74)
Warranty	(2)	(2)	(3)
Other	(27)	(4)	(19)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (141)	$ (136)	$ (196)