Segment Information (Details) - Net Written Premiums - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Net written premiums	$ 43,356	$ 40,201	$ 35,414
Business Insurance [Member]
Segment reporting information
Net written premiums	22,078	20,430	17,635
Business Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	20,143	18,665	16,574
Business Insurance [Member] | Domestic [Member] | Select Accounts [Member]
Segment reporting information
Net written premiums	3,727	3,477	3,099
Business Insurance [Member] | Domestic [Member] | Middle Market [Member]
Segment reporting information
Net written premiums	12,023	11,045	9,923
Business Insurance [Member] | Domestic [Member] | National Accounts [Member]
Segment reporting information
Net written premiums	1,259	1,135	1,085
Business Insurance [Member] | Domestic [Member] | National Property and Other [Member]
Segment reporting information
Net written premiums	3,134	3,008	2,467
Business Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	1,935	1,765	1,061
Bond & Specialty Insurance [Member]
Segment reporting information
Net written premiums	4,109	3,842	3,732
Bond & Specialty Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	3,603	3,303	3,193
Bond & Specialty Insurance [Member] | Domestic [Member] | Management Liability [Member]
Segment reporting information
Net written premiums	2,309	2,156	2,112
Bond & Specialty Insurance [Member] | Domestic [Member] | Surety [Member]
Segment reporting information
Net written premiums	1,294	1,147	1,081
Bond & Specialty Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	506	539	539
Personal Insurance [Member]
Segment reporting information
Net written premiums	17,169	15,929	14,047
Personal Insurance [Member] | Domestic [Member]
Segment reporting information
Net written premiums	16,475	15,279	13,398
Personal Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Net written premiums	7,925	7,330	6,482
Personal Insurance [Member] | Domestic [Member] | Homeowners and Other [Member]
Segment reporting information
Net written premiums	8,550	7,949	6,916
Personal Insurance [Member] | International [Member]
Segment reporting information
Net written premiums	$ 694	$ 650	$ 649