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
	Mar. 31, 2019	Mar. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ (5)	$ (1)
General Liability	(20)	(8)
Workers' Compensation	2	(6)
Property and Other	15	6
Total pretax (favorable) unfavorable development	$ (8)	$ (9)