Segment Information (details) - Assets by Segment - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets by segment
Total assets	$ 104,311	$ 100,245
Reportable Segments [Member]
Assets by segment
Total assets	103,952	99,882
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	78,985	75,730
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	9,005	8,726
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	15,962	15,426
Other assets [Member]
Assets by segment
Total assets	$ 359	$ 363