Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment reporting information
Net written premiums	$ 31,959	$ 29,732	$ 29,151
Business Insurance [Member]
Segment reporting information
Net written premiums	16,092	15,431	15,629
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	15,018	14,414	14,557
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	2,833	2,821	2,911
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	8,933	8,511	8,630
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	987	996	1,051
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	2,265	2,086	1,965
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,074	1,017	1,072
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	3,376	2,951	2,739
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	2,871	2,614	2,471
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	1,983	1,769	1,605
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	888	845	866
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	505	337	268
Personal Insurance [Member]
Segment reporting information
Net written premiums	12,491	11,350	10,783
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	11,807	10,698	10,076
Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Net written premiums	5,827	5,369	5,412
Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Net written premiums	5,980	5,329	4,664
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 684	$ 652	$ 707