Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment reporting information
Net written premiums	$ 27,708	$ 26,219	$ 24,958
Business Insurance [Member]
Segment reporting information
Net written premiums	14,956	14,270	13,900
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	13,872	13,257	12,945
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,828	2,800	2,729
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	8,214	7,756	7,379
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,025	1,010	1,058
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	1,805	1,691	1,779
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,084	1,013	955
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	2,528	2,359	2,271
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	2,290	2,160	2,099
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	1,455	1,367	1,342
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	835	793	757
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	238	199	172
Personal Insurance [Member]
Segment reporting information
Net written premiums	10,224	9,590	8,787
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	9,516	8,940	8,184
Personal Insurance [Member] | Domestic [Member] | Total Agency [Member]
Segment reporting information
Net written premiums	9,120	8,579	7,875
Personal Insurance [Member] | Domestic [Member] | Agency Automobile [Member]
Segment reporting information
Net written premiums	4,972	4,646	4,103
Personal Insurance [Member] | Domestic [Member] | Agency Homeowners and Other [Member]
Segment reporting information
Net written premiums	4,148	3,933	3,772
Personal Insurance [Member] | Domestic [Member] | Direct-to-Consumer [Member]
Segment reporting information
Net written premiums	396	361	309
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 708	$ 650	$ 603