Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets by segment
Total assets	$ 109,436	$ 110,122
Reportable Segments [Member]
Assets by segment
Total assets	108,881	109,510
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	83,525	83,896
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	8,690	8,599
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	16,666	17,015
Other assets [Member]
Assets by segment
Total assets	$ 555	$ 612