Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets by segment
Total assets	$ 104,390	$ 103,483
Reportable Segments [Member]
Assets by segment
Total assets	103,680	102,807
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	78,665	78,082
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	9,060	8,776
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	15,955	15,949
Other assets [Member]
Assets by segment
Total assets	$ 710	$ 676