Claim and Claim Adjustment Expense Reserves (Commercial - Detail of Develpment Recorded) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (5)	$ (17)	$ (27)	$ (94)
Commercial Auto
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	1	(12)	0	(37)
General Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(5)	(2)	13	(19)
Workers' Compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(2)	9	(14)	(38)
Property and Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ 1	$ (12)	$ (26)	$ 0