Property Casualty Loss and Loss Expenses - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	$ 11,260		$ 11,260		$ 10,003
Prior accident years	(22)	$ (71)	(176)	$ (211)
Life Insurance Segment
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Reserve for loss and loss expenses	65	62	65	62
Commercial Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(18)	(50)	(103)	(117)
Commercial Insurance \| Commercial Property Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(38)	(33)	(113)	(76)
Commercial Insurance \| Workers' Compensation Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	(17)	(16)	(45)	(56)
Commercial Insurance \| Commercial Casualty Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	24		21	27
Commercial Insurance \| Commercial Auto Line
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	10		35	(10)
Personal Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	14	1	(24)	(27)
Personal Insurance \| Homeowner
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years			(47)	(37)
Excess and Surplus Lines Insurance
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters
Prior accident years	$ (4)	$ 5	$ (18)	$ 5

CINCINNATI FINANCIAL CORP 10-Q