Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ 29	$ 22	$ 14	$ 7
Property and Casualty Operations [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	11	28	26	43
Medical Professional Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development			8	10
Other Professional Liability and Management Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	10	(9)	10	(6)
Surety [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(23)		(38)	(30)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	30	15	30	24
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(42)	(61)	(42)	(74)
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	14	27	6	33
Other Insurance Operations [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ 40	$ 50	$ 40	$ 50