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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 9	$ (16)	$ 33	$ (24)
General Liability	15	43	65	36
Workers' Compensation	(23)	7	(97)	2
Property and Other	0	1	41	1
Total pretax (favorable) unfavorable development	$ 1	$ 35	$ 42	$ 15