Claim and Claim Adjustment Expense Reserves (International - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (308)	$ (288)	$ (210)
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	(4)	(5)	(9)
Other Professional Liability	(13)	12	(16)
Liability	2	(30)	(17)
Property & Marine	(15)	(34)	(29)
Other	21	(1)	17
Total pretax (favorable) unfavorable development	$ (9)	$ (58)	$ (54)