Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ (10)	$ (15)	$ 4	$ (8)
Property and Casualty Operations [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(10)	(15)	(36)	(58)
Medical Professional Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	8	25	16	35
Other Professional Liability and Management Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development			10	(6)
Surety [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(15)	(40)	(53)	(70)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		9	30	33
General Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		15		15
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	2	(23)	(40)	(97)
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ (5)	$ (1)	1	32
Other Insurance Operations [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development			$ 40	$ 50