Segment Information (details) - Assets by Segment (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets by segment
Total assets	$ 103,897	$ 104,938
Total reportable segments [Member]
Assets by segment
Total assets	103,585	104,619
Business Insurance [Member]
Assets by segment
Total assets	76,583	76,972
Financial, Professional & International Insurance [Member]
Assets by segment
Total assets	13,318	13,452
Personal Insurance [Member]
Assets by segment
Total assets	13,684	14,195
Other [Member]
Assets by segment
Total assets	$ 312	$ 319