Claim and Claim Adjustment Expense Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Commercial - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 1	$ (35)	$ (47)
General Liability	32	(24)	(65)
Workers' Compensation	(32)	(63)	145
Property and Other	(26)	7	(18)
Total pretax (favorable) unfavorable development	$ (25)	$ (115)	$ 15