Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	$ 14,989	$ 14,270	$ 13,299
Total operating revenues	15,070	14,351	13,398
Net investment gains (losses)	(81)	(81)	(99)
Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	5,701	5,598	5,490
Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	6,502	5,920	5,221
Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,467	1,387	1,279
Operating Segments | Life & Group
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,337	1,377	1,346
Operating Segments | Corporate & Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	74	80	73
Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	(11)	(11)	(11)
Management & Professional Liability | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	3,102	2,998	2,931
Surety | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	835	785	731
Warranty & Alternative Risks | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,764	1,815	1,828
Middle Market | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,925	1,775	1,696
Construction | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	2,088	1,991	1,678
Small Business | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	639	637	631
Other Commercial | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,850	1,517	1,216
Canada | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	422	401	383
Europe | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	627	593	532
Hardy | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	$ 418	$ 393	$ 364