Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Goodwill by segment
Goodwill	$ 4,251	$ 3,976
Reportable Segments [Member] | Business Insurance [Member]
Goodwill by segment
Goodwill	2,579	2,585
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Goodwill by segment
Goodwill	835	550
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Corvus [Member]
Goodwill by segment
Goodwill	285
Amount of goodwill expected to be deductible for tax purposes	0
Reportable Segments [Member] | Personal Insurance [Member]
Goodwill by segment
Goodwill	811	815
Other [Member]
Goodwill by segment
Goodwill	$ 26	$ 26