Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022		Jun. 30, 2023	Jun. 30, 2022
Total revenues	$ 3,304	$ 2,926	[1]	$ 6,456	$ 5,811
Total operating revenues	3,336	2,985		6,523	5,881
Net investment gains (losses)	(32)	(59)	[1]	(67)	(70)	[1]
Operating Segments | Specialty
Total revenues	1,361	1,286		2,694	2,544
Operating Segments | Commercial
Total revenues	1,291	1,091		2,493	2,121
Operating Segments | International
Total revenues	328	284		641	561
Operating Segments | Life & Group
Total revenues	342	320		671	651
Operating Segments | Corporate & Other
Total revenues	16	5		29	7
Eliminations
Total revenues	(2)	(1)		(5)	(3)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	718	680		1,423	1,353
Surety | Operating Segments | Specialty
Total revenues	183	162		351	310
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	460	444		920	881
Middle Market | Operating Segments | Commercial
Total revenues	419	374		817	736
Construction | Operating Segments | Commercial
Total revenues	420	350		805	674
Small Business | Operating Segments | Commercial
Total revenues	160	143		310	281
Other Commercial | Operating Segments | Commercial
Total revenues	292	224		561	430
Canada | Operating Segments | International
Total revenues	96	91		189	179
Europe | Operating Segments | International
Total revenues	135	118		262	238
Hardy | Operating Segments | International
Total revenues	$ 97	$ 75		$ 190	$ 144

[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.