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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ 22	$ (31)	$ 7	$ (45)
Medical Professional Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		15	10	30
Other Professional Liability and Management Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(9)	(7)	(6)	(19)
Surety [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development		(15)	(30)	(40)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	15	(3)	24	(8)
General Liability [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	50	13	50	(7)
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	(61)	(7)	(74)	(5)
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax (favorable) unfavorable development	$ 27	$ (27)	$ 33	$ 4