Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets by segment
Total assets	$ 110,122	$ 104,233
Reportable segments [Member]
Assets by segment
Total assets	109,510	103,601
Reportable segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	83,896	78,965
Reportable segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	8,599	8,693
Reportable segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	17,015	15,943
Other assets [Member]
Assets by segment
Total assets	$ 612	$ 632