Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment reporting information
Net written premiums	$ 40,201	$ 35,414	$ 31,959
Business Insurance [Member]
Segment reporting information
Net written premiums	20,430	17,635	16,092
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	18,665	16,574	15,018
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	3,477	3,099	2,833
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	11,045	9,923	8,933
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,135	1,085	987
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	3,008	2,467	2,265
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,765	1,061	1,074
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	3,842	3,732	3,376
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	3,303	3,193	2,871
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	2,156	2,112	1,983
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	1,147	1,081	888
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	539	539	505
Personal Insurance [Member]
Segment reporting information
Net written premiums	15,929	14,047	12,491
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	15,279	13,398	11,807
Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Net written premiums	7,330	6,482	5,827
Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Net written premiums	7,949	6,916	5,980
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 650	$ 649	$ 684