Claim and Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ (16)	$ 1	$ (24)	$ 0
General Liability	43	(5)	36	13
Workers' Compensation	7	(2)	2	(14)
Property and Other	1	1	1	(26)
Total pretax (favorable) unfavorable development	$ 35	$ (5)	$ 15	$ (27)