Claim and Claim Adjustment Expense Reserves (Commercial - 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)
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	(38)	(46)	(22)
General Liability	0	(28)	(33)
Workers' Compensation	(65)	150	80
Property and Other	16	(21)	(40)
Total pretax (favorable) unfavorable development	$ (87)	$ 55	$ (15)