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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 0	$ 9	$ 30	$ 33
General Liability	0	15	0	15
Workers' Compensation	2	(23)	(40)	(97)
Property and Other	0	0	12	41
Total pretax (favorable) unfavorable development	$ 2	$ 1	$ 2	$ (8)