Goodwill and Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Goodwill by segment
Goodwill	$ 4,017	$ 3,976
Reportable Segments [Member] | Business Insurance [Member]
Goodwill by segment
Goodwill	2,618	2,613
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Goodwill by segment
Goodwill	550	550
Reportable Segments [Member] | Personal Insurance [Member]
Goodwill by segment
Goodwill	823	787
Reportable Segments [Member] | Personal Insurance [Member] | Business Acquisition [Member]
Goodwill by segment
Goodwill	33
Other [Member]
Goodwill by segment
Goodwill	$ 26	$ 26