Segment Information (Details) - Prior Year Development and Catastrophes - Reportable segments [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment reporting information
Net favorable (unfavorable) prior year reserve development included in claims and claim adjustment expenses	$ 709	$ 143	$ 649
Catastrophe losses included in claims and claim adjustment expenses	3,335	2,991	1,877
Business Insurance [Member]
Segment reporting information
Net favorable (unfavorable) prior year reserve development included in claims and claim adjustment expenses	90	(289)	381
Catastrophe losses included in claims and claim adjustment expenses	1,032	838	654
Bond & Specialty Insurance [Member]
Segment reporting information
Net favorable (unfavorable) prior year reserve development included in claims and claim adjustment expenses	129	285	222
Catastrophe losses included in claims and claim adjustment expenses	51	37	25
Personal Insurance [Member]
Segment reporting information
Net favorable (unfavorable) prior year reserve development included in claims and claim adjustment expenses	490	147	46
Catastrophe losses included in claims and claim adjustment expenses	$ 2,252	$ 2,116	$ 1,198