Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting [Line Items]
Total revenues	$ 3,829	$ 3,717	$ 7,506	$ 7,344
Total operating revenues	3,834	3,763	7,529	7,399
Net investment gains (losses)	(5)	(46)	(23)	(55)
Operating Segments | Specialty
Segment Reporting [Line Items]
Total revenues	1,416	1,429	2,785	2,808
Operating Segments | Commercial
Segment Reporting [Line Items]
Total revenues	1,695	1,618	3,306	3,183
Operating Segments | International
Segment Reporting [Line Items]
Total revenues	381	362	758	706
Operating Segments | Life & Group
Segment Reporting [Line Items]
Total revenues	332	341	659	673
Operating Segments | Corporate & Other
Segment Reporting [Line Items]
Total revenues	13	16	27	35
Eliminations
Segment Reporting [Line Items]
Total revenues	(3)	(3)	(6)	(6)
Management & Professional Liability | Operating Segments | Specialty
Segment Reporting [Line Items]
Total revenues	802	778	1,563	1,524
Surety | Operating Segments | Specialty
Segment Reporting [Line Items]
Total revenues	208	205	399	393
Warranty & Alternative Risks | Operating Segments | Specialty
Segment Reporting [Line Items]
Total revenues	406	446	823	891
Middle Markets | Operating Segments | Commercial
Segment Reporting [Line Items]
Total revenues	509	468	988	921
Construction | Operating Segments | Commercial
Segment Reporting [Line Items]
Total revenues	512	514	1,013	1,020
Small Business | Operating Segments | Commercial
Segment Reporting [Line Items]
Total revenues	157	161	308	317
Other Commercial | Operating Segments | Commercial
Segment Reporting [Line Items]
Total revenues	517	475	997	925
Canada | Operating Segments | International
Segment Reporting [Line Items]
Total revenues	114	104	221	198
Europe | Operating Segments | International
Segment Reporting [Line Items]
Total revenues	166	156	335	300
Hardy | Operating Segments | International
Segment Reporting [Line Items]
Total revenues	$ 101	$ 102	$ 202	$ 208