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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 15	$ (3)	$ 24	$ (8)
General Liability	50	13	50	(7)
Workers' Compensation	(61)	(7)	(74)	(5)
Property and Other	41	(15)	41	0
Total pretax (favorable) unfavorable development	$ 45	$ (12)	$ 41	$ (20)