Claim and Claim Adjustment Expense Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ (3)	$ 0	$ (8)	$ (1)
General Liability	13	26	(7)	18
Workers' Compensation	(7)	(6)	(5)	(12)
Property and Other	(15)	(33)	0	(27)
Total pretax (favorable) unfavorable development	$ (12)	$ (13)	$ (20)	$ (22)