Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets by segment
Total assets	$ 107,246	$ 104,233
Reportable Segments [Member]
Assets by segment
Total assets	106,673	103,601
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	82,022	78,965
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	8,363	8,693
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	16,288	15,943
Other assets [Member]
Assets by segment
Total assets	$ 573	$ 632