Segment Information (Details) - Assets by Segment - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets by segment
Total assets	$ 120,706	$ 116,764
Reportable Segments [Member]
Assets by segment
Total assets	120,002	116,170
Reportable Segments [Member] | Business Insurance [Member]
Assets by segment
Total assets	90,661	88,422
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Assets by segment
Total assets	10,313	9,420
Reportable Segments [Member] | Personal Insurance [Member]
Assets by segment
Total assets	19,028	18,328
Other assets [Member]
Assets by segment
Total assets	$ 704	$ 594