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
	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)
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	(26)	(15)
General Liability	0	(15)
Workers' Compensation	0	4
Property and Other	2	12
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (24)	$ (14)