Business Segments (Revenues by line of business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Total revenues	$ 2,957	$ 2,959	$ 8,768	$ 8,854
Total operating revenues	3,053	2,937	8,934	8,737
Net investment gains (losses)	(96)	22	(166)	117
Operating Segments | Specialty
Total revenues	1,310	1,247	3,854	3,692
Operating Segments | Commercial
Total revenues	1,148	1,041	3,269	3,109
Operating Segments | International
Total revenues	286	285	847	832
Operating Segments | Life & Group
Total revenues	305	362	956	1,092
Operating Segments | Corporate & Other
Total revenues	6	4	13	17
Eliminations
Total revenues	(2)	(2)	(5)	(5)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	688	684	2,041	2,044
Surety | Operating Segments | Specialty
Total revenues	171	157	481	454
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	451	406	1,332	1,194
Middle Market | Operating Segments | Commercial
Total revenues	397	363	1,133	1,119
Construction | Operating Segments | Commercial
Total revenues	372	345	1,046	978
Small Business | Operating Segments | Commercial
Total revenues	149	144	430	413
Other Commercial | Operating Segments | Commercial
Total revenues	230	189	660	599
Canada | Operating Segments | International
Total revenues	93	87	272	253
Europe | Operating Segments | International
Total revenues	112	123	350	349
Hardy | Operating Segments | International
Total revenues	$ 81	$ 75	$ 225	$ 230