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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 30	$ 15	$ 30	$ 24
General Liability	0	0	0	0
Workers' Compensation	(42)	(61)	(42)	(74)
Property and Other	12	41	12	41
Total pretax (favorable) unfavorable development	$ 0	$ (5)	$ 0	$ (9)