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, 2016	Dec. 31, 2015	Dec. 31, 2014
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (288)	$ (210)	$ (21)
Specialty [Member]
Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability	(37)	(43)	39
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability	(130)	0	(87)
Net prior year claim and allocated claim adjustment expense reserve development, Surety	(63)	(69)	(82)
Warranty	4	(2)	(2)
Net prior year claim and allocated claim adjustment expense reserve development, Other	(61)	(27)	(4)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (287)	$ (141)	$ (136)