Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Segment Reconciling Items | Asbestos and Environmental
Prior accident year development	$ 0	$ 0
Segment Reconciling Items | Catastrophe
Prior accident year development	(44)	(32)
Accident and Health Insurance Product Line [Member] | Segment Reconciling Items
Prior accident year development	(174)	(143)
Accident and Health Insurance Product Line [Member] | Segment Reconciling Items | COVID-19 related Claims
Prior accident year development	38	14
Change in Workers Compensation Discount Including Accretion [Member] | Segment Reconciling Items
Prior accident year development	32	27
General Liability [Member] | Segment Reconciling Items
Prior accident year development	39	33
Marine | Segment Reconciling Items
Prior accident year development	(1)	2
Package Business [Member] | Segment Reconciling Items
Prior accident year development	(18)	(35)
Property Insurance [Member] | Commercial Lines | Segment Reconciling Items
Prior accident year development	2	(17)
Property Insurance [Member] | Personal Lines | Segment Reconciling Items
Prior accident year development	1	(2)
Professional Liability Insurance [Member] | Segment Reconciling Items
Prior accident year development	(3)	(9)
Surety Product Line [Member] | Segment Reconciling Items
Prior accident year development	12	(4)
Assumed reinsurance | Segment Reconciling Items
Prior accident year development	19	12
Automobiles [Member] | Commercial Lines | Segment Reconciling Items
Prior accident year development	6	23
Automobiles [Member] | Personal Lines | Segment Reconciling Items
Prior accident year development	0	(14)
Uncollectible reinsurance | Segment Reconciling Items
Prior accident year development	13	3
Insurance, Other [Member] | Segment Reconciling Items
Prior accident year development	34	9
Property, Liability and Casualty Insurance Product Line
Prior accident year development	$ (82)	$ (147)