Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment reporting information
Net written premiums	$ 35,414	$ 31,959	$ 29,732
Business Insurance [Member]
Segment reporting information
Net written premiums	17,635	16,092	15,431
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	16,574	15,018	14,414
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	3,099	2,833	2,821
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	9,923	8,933	8,511
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,085	987	996
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	2,467	2,265	2,086
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,061	1,074	1,017
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	3,732	3,376	2,951
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	3,193	2,871	2,614
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	2,112	1,983	1,769
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	1,081	888	845
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	539	505	337
Personal Insurance [Member]
Segment reporting information
Net written premiums	14,047	12,491	11,350
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	13,398	11,807	10,698
Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Net written premiums	6,482	5,827	5,369
Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Net written premiums	6,916	5,980	5,329
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 649	$ 684	$ 652