Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ (15)	$ (15)
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		3
Surety [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(15)	(30)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		9
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		(13)
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ (8)	$ 6