Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (132)	$ (175)	$ (282)	$ (187)
Specialty [Member]
Medical Professional Liability	13	(19)	(17)	(11)
Other Professional Liability and Management Liability	(48)	(37)	(98)	(41)
Surety	(63)	(70)	(63)	(69)
Warranty	2	0	7	1
Other	(16)	(4)	(40)	(21)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (112)	$ (130)	$ (211)	$ (141)