Claim and Claim Adjustment Expense Reserves (Commercial - Detail of Develpment Recorded) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (13)	$ (34)	$ (22)	$ (77)
Commercial Auto
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	0	1	(1)	(25)
General Liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	26	1	18	(17)
Workers' Compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	(6)	(47)	(12)	(47)
Property and Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (33)	$ 11	$ (27)	$ 12