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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ (115)	$ (132)	$ (227)	$ (282)
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	4	(2)	(1)	(3)
Other Professional Liability	(8)	(1)	1	16
Liability	4	(2)	(1)	(21)
Property & Marine	(4)	(9)	(15)	(16)
Other	5	(1)	17	(10)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ 1	$ (15)	$ 1	$ (34)