Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022		Sep. 30, 2023	Sep. 30, 2022
Total revenues	$ 3,336	$ 2,957	[1]	$ 9,792	$ 8,768	[1]
Total operating revenues	3,374	3,053		9,897	8,934
Net investment gains (losses)	(38)	(96)	[1]	(105)	(166)	[1]
Operating Segments | Specialty
Total revenues	1,372	1,310		4,066	3,854
Operating Segments | Commercial
Total revenues	1,335	1,148		3,828	3,269
Operating Segments | International
Total revenues	321	286		962	847
Operating Segments | Life & Group
Total revenues	328	305		999	956
Operating Segments | Corporate & Other
Total revenues	21	6		50	13
Eliminations
Total revenues	(3)	(2)		(8)	(5)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	723	688		2,146	2,041
Surety | Operating Segments | Specialty
Total revenues	189	171		540	481
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	460	451		1,380	1,332
Middle Market | Operating Segments | Commercial
Total revenues	437	397		1,254	1,133
Construction | Operating Segments | Commercial
Total revenues	444	372		1,249	1,046
Small Business | Operating Segments | Commercial
Total revenues	160	149		470	430
Other Commercial | Operating Segments | Commercial
Total revenues	294	230		855	660
Canada | Operating Segments | International
Total revenues	98	93		287	272
Europe | Operating Segments | International
Total revenues	139	112		401	350
Hardy | Operating Segments | International
Total revenues	$ 84	$ 81		$ 274	$ 225

[1] As of January 1, 2023, the Company adopted ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts (ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance. See Note A to the Condensed Consolidated Financial Statements for additional information.