Goodwill and Other Intangible Assets (Details) - Goodwill - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Goodwill by segment
Goodwill	$ 4,233	$ 3,976
Reportable segments [Member] | Business Insurance [Member]
Goodwill by segment
Goodwill	2,572	2,585
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Goodwill by segment
Goodwill	834	550
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Corvus [Member]
Goodwill by segment
Goodwill	284
Amount of goodwill expected to be deductible for tax purposes	0
Reportable segments [Member] | Personal Insurance [Member]
Goodwill by segment
Goodwill	801	815
Other [Member]
Goodwill by segment
Goodwill	$ 26	$ 26