Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets by segment
Total assets	$ 116,384	$ 110,122
Reportable Segments [Member]
Assets by segment
Total assets	115,753	109,510
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	88,040	83,896
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	9,483	8,599
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	18,230	17,015
Other assets [Member]
Assets by segment
Total assets	$ 631	$ 612