Segment Information (details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment reporting information
Net written premiums	$ 26,219	$ 24,958	$ 24,121
Business Insurance [Member]
Segment reporting information
Net written premiums	14,270	13,900	13,774
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	13,257	12,945	12,741
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,800	2,729	2,716
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	7,756	7,379	7,186
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,010	1,058	1,048
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	1,691	1,779	1,791
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,013	955	1,033
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	2,359	2,271	2,273
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	2,160	2,099	2,081
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	1,367	1,342	1,326
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	793	757	755
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	199	172	192
Personal Insurance [Member]
Segment reporting information
Net written premiums	9,590	8,787	8,074
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	8,940	8,184	7,457
Personal Insurance [Member] | Domestic [Member] | Total Agency [Member]
Segment reporting information
Net written premiums	8,579	7,875	7,221
Personal Insurance [Member] | Domestic [Member] | Agency Automobile [Member]
Segment reporting information
Net written premiums	4,646	4,103	3,534
Personal Insurance [Member] | Domestic [Member] | Agency Homeowners and Other [Member]
Segment reporting information
Net written premiums	3,933	3,772	3,687
Personal Insurance [Member] | Domestic [Member] | Direct-to-Consumer [Member]
Segment reporting information
Net written premiums	361	309	236
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 650	$ 603	$ 617