Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment reporting information
Net written premiums	$ 29,151	$ 27,708	$ 26,219
Business Insurance [Member]
Segment reporting information
Net written premiums	15,629	14,956	14,270
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	14,557	13,872	13,257
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,911	2,828	2,800
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	8,630	8,214	7,756
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,051	1,025	1,010
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	1,965	1,805	1,691
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,072	1,084	1,013
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	2,739	2,528	2,359
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	2,471	2,290	2,160
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	1,605	1,455	1,367
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	866	835	793
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	268	238	199
Personal Insurance [Member]
Segment reporting information
Net written premiums	10,783	10,224	9,590
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	10,076	9,516	8,940
Personal Insurance [Member] | Domestic [Member] | Total Agency [Member]
Segment reporting information
Net written premiums	9,664	9,120	8,579
Personal Insurance [Member] | Domestic [Member] | Agency Automobile [Member]
Segment reporting information
Net written premiums	5,124	4,972	4,646
Personal Insurance [Member] | Domestic [Member] | Agency Homeowners and Other [Member]
Segment reporting information
Net written premiums	4,540	4,148	3,933
Personal Insurance [Member] | Domestic [Member] | Direct-to-Consumer [Member]
Segment reporting information
Net written premiums	412	396	361
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 707	$ 708	$ 650