Claim and Claim Adjustment Expense Reserves (Net Prior Year Development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ 48	$ (32)	$ 11
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(14)	(40)	(45)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(22)	(43)	(6)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	13	(13)	2
Corporate & Other
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ 71	$ 64	$ 60