Claim and Claim Adjustment Expense Reserves - Net prior year claim and allocated claim adjustment expense reserve development for International segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (55)	$ (98)	$ (112)	$ (150)
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	(5)	(1)	(5)	(1)
Other Professional Liability	10	18	9	17
Liability	(5)	(19)	(5)	(19)
Property & Marine	(12)	(3)	(11)	(7)
Other	14	(10)	12	(9)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ 2	$ (15)	$ 0	$ (19)