Property Casualty Loss and Loss Expenses - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2025
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 12,750		$ 12,750		$ 11,507
Prior accident years	(94)	$ (22)	(217)	$ (176)
Life Insurance Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	76	65	76	65
Commercial Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(46)	(18)	(116)	(103)
Commercial Insurance \| Commercial Property Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(22)	(38)	(72)	(113)
Commercial Insurance \| Workers' Compensation Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(17)	(17)	(40)	(45)
Commercial Insurance \| Commercial Auto Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		10		35
Commercial Insurance \| Commercial Casualty Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years		24		21
Personal Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(38)	14	(56)	(24)
Personal Insurance \| Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(33)		(52)	(47)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ (6)	$ (4)	$ (20)	$ (18)