Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets by segment
Total assets	$ 116,764	$ 110,122
Reportable segments [Member]
Assets by segment
Total assets	116,170	109,510
Reportable segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	88,422	83,896
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	9,420	8,599
Reportable segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	18,328	17,015
Other assets [Member]
Assets by segment
Total assets	$ 594	$ 612