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
	Mar. 31, 2020	Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 9	$ (5)
General Liability	0	(20)
Workers' Compensation	(13)	2
Property and Other	0	15
Total pretax (favorable) unfavorable development	$ (4)	$ (8)