Segment Information (details) - Net Written Premiums (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment reporting information
Net written premiums	$ 22,767	$ 22,447	$ 22,187
Reportable Segments [Member]
Segment reporting information
Net written premiums	22,767	22,447	22,187
Reportable Segments [Member] | Business Insurance [Member]
Segment reporting information
Net written premiums	12,233	11,872	11,340
Reportable Segments [Member] | Business Insurance [Member] | Total Business Insurance Core [Member]
Segment reporting information
Net written premiums	12,233	11,873	11,330
Reportable Segments [Member] | Business Insurance [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,724	2,775	2,784
Reportable Segments [Member] | Business Insurance [Member] | Commercial Accounts [Member]
Segment reporting information
Net written premiums	3,197	3,101	2,890
Reportable Segments [Member] | Business Insurance [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,010	907	782
Reportable Segments [Member] | Business Insurance [Member] | Industry-Focused Underwriting [Member]
Segment reporting information
Net written premiums	2,645	2,554	2,407
Reportable Segments [Member] | Business Insurance [Member] | Target Risk Underwriting [Member]
Segment reporting information
Net written premiums	1,799	1,666	1,587
Reportable Segments [Member] | Business Insurance [Member] | Specialized Distribution [Member]
Segment reporting information
Net written premiums	858	870	880
Reportable Segments [Member] | Business Insurance [Member] | Other [Member]
Segment reporting information
Net written premiums		(1)	10
Reportable Segments [Member] | Financial, Professional & International Insurance [Member]
Segment reporting information
Net written premiums	3,309	2,981	3,102
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | Bond and Financial Products [Member]
Segment reporting information
Net written premiums	2,030	1,924	1,953
Reportable Segments [Member] | Financial, Professional & International Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,279	1,057	1,149
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Net written premiums	7,225	7,594	7,745
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Net written premiums	3,370	3,642	3,788
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Net written premiums	$ 3,855	$ 3,952	$ 3,957