Segment Information (details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment reporting information
Net written premiums	$ 24,121	$ 23,904	$ 22,767
Business and International Insurance [Member]
Segment reporting information
Net written premiums	14,583	14,636	13,512
Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	12,764	12,515	12,233
Business and International Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,716	2,707	2,724
Business and International Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	6,325	6,108	5,862
Business and International Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,048	1,047	1,010
Business and International Insurance [Member] | Domestic [Member] | First Party [Member]
Segment reporting information
Net written premiums	1,564	1,579	1,552
Business and International Insurance [Member] | Domestic [Member] | Specialized Distribution [Member]
Segment reporting information
Net written premiums	1,111	1,074	1,085
Business and International Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,819	2,121	1,279
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	2,081	2,103	2,030
Personal Insurance [Member]
Segment reporting information
Net written premiums	7,457	7,165	7,225
Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Net written premiums	3,700	3,390	3,370
Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Net written premiums	$ 3,757	$ 3,775	$ 3,855