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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ (55)	$ (98)	$ (112)	$ (150)
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	0	(20)	(26)	(35)
General Liability	(1)	(37)	(1)	(52)
Workers' Compensation	(46)	50	(46)	54
Property and Other	13	(11)	15	1
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (34)	$ (18)	$ (58)	$ (32)