Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets by segment
Total assets	$ 108,572	$ 104,233
Reportable Segments [Member]
Assets by segment
Total assets	107,809	103,601
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	82,762	78,965
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	8,445	8,693
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	16,602	15,943
Other assets [Member]
Assets by segment
Total assets	$ 763	$ 632