Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment reporting information
Net written premiums	$ 29,732	$ 29,151	$ 27,708
Business Insurance [Member]
Segment reporting information
Net written premiums	15,431	15,629	14,956
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	14,414	14,557	13,872
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,821	2,911	2,828
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	8,511	8,630	8,214
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	996	1,051	1,025
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	2,086	1,965	1,805
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,017	1,072	1,084
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	2,951	2,739	2,528
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	2,614	2,471	2,290
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	1,769	1,605	1,455
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	845	866	835
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	337	268	238
Personal Insurance [Member]
Segment reporting information
Net written premiums	11,350	10,783	10,224
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	10,698	10,076	9,516
Personal Insurance [Member] | Domestic [Member] | Total Agency [Member]
Segment reporting information
Net written premiums	10,265	9,664	9,120
Personal Insurance [Member] | Domestic [Member] | Agency Automobile [Member]
Segment reporting information
Net written premiums	5,080	5,124	4,972
Personal Insurance [Member] | Domestic [Member] | Agency Homeowners and Other [Member]
Segment reporting information
Net written premiums	5,185	4,540	4,148
Personal Insurance [Member] | Domestic [Member] | Direct-to-Consumer [Member]
Segment reporting information
Net written premiums	433	412	396
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 652	$ 707	$ 708