Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ (15)	$ (14)
Medical Professional Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	10	15
Other Professional Liability and Management Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	3	(12)
Surety [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(30)	(25)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	9	(5)
General Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		(20)
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(13)	2
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ 6	$ 31