Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets by segment
Total assets	$ 113,337	$ 110,122
Reportable Segments [Member]
Assets by segment
Total assets	112,718	109,510
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	86,136	83,896
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	9,155	8,599
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	17,427	17,015
Other assets [Member]
Assets by segment
Total assets	$ 619	$ 612