Claim and Claim Adjustment Expense Reserves - Net prior year claim and allocated claim adjustment expense reserve development for Commercial 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)
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	(14)	(12)	(40)	(47)
General Liability	7	14	6	(38)
Workers' Compensation	7	(6)	(39)	48
Property and Other	(7)	(1)	8	0
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (7)	$ (5)	$ (65)	$ (37)