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
	Mar. 31, 2017	Mar. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (57)	$ (52)
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	0	0
Medical Professional Liability	(1)	(1)
Other Professional Liability	0	0
Liability	1	(4)
Other	(2)	1
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (2)	$ (4)