Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Net Prior Year Loss Reserve Development in Property and Casualty Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Prior Year Development [Abstract]
Pretax favorable development	$ (20)	$ (73)	$ (181)
Medical Professional Liability [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	35	75	47
Other Professional Liability and Management Liability [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	(15)	(69)	(127)
Surety [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	(69)	(92)	(70)
Commercial Auto [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	33	(25)	1
General Liability [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	65	54	32
Workers' Compensation [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	(96)	(13)	(32)
Property and Other [Member]
Net Prior Year Development [Abstract]
Pretax favorable development	$ 27	$ (3)	$ (32)