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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (98)	$ (5)	$ (150)	$ (12)
Specialty [Member]
Medical Professional Liability	(23)	(6)	(30)	8
Other Professional Liability and Management Liability	(41)	(1)	(50)	(4)
Surety	0	0	0	1
Warranty	3	1	5	1
Other	(4)	(7)	(24)	(17)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (65)	$ (13)	$ (99)	$ (11)