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, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ (25)	$ 1	$ (35)
General Liability	54	32	(24)
Workers' Compensation	(13)	(32)	(63)
Property and Other	(18)	(26)	7
Total pretax (favorable) unfavorable development	$ (2)	$ (25)	$ (115)