Segment Information (details) - Assets by Segment (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Assets by segment
Total assets	$ 104,938	$ 104,575
Total reportable segments [Member]
Assets by segment
Total assets	104,619	103,878
Business Insurance [Member]
Assets by segment
Total assets	76,972	76,909
Financial, Professional & International Insurance [Member]
Assets by segment
Total assets	13,452	13,355
Personal Insurance [Member]
Assets by segment
Total assets	14,195	13,614
Other [Member]
Assets by segment
Total assets	$ 319	$ 697