Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets by segment
Total assets	$ 110,241	$ 104,233
Reportable Segments [Member]
Assets by segment
Total assets	109,529	103,601
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	83,876	78,965
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	8,708	8,693
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	16,945	15,943
Other assets [Member]
Assets by segment
Total assets	$ 712	$ 632