Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Total revenues	$ 3,444	$ 3,152
Total operating revenues	3,466	3,187
Net investment gains (losses)	(22)	(35)
Operating Segments | Specialty
Total revenues	1,372	1,333
Operating Segments | Commercial
Total revenues	1,386	1,202
Operating Segments | International
Total revenues	346	313
Operating Segments | Life & Group
Total revenues	341	329
Operating Segments | Corporate & Other
Total revenues	24	13
Eliminations
Total revenues	(3)	(3)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	729	705
Surety | Operating Segments | Specialty
Total revenues	182	168
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	461	460
Middle Market | Operating Segments | Commercial
Total revenues	432	398
Construction | Operating Segments | Commercial
Total revenues	455	385
Small Business | Operating Segments | Commercial
Total revenues	154	150
Other Commercial | Operating Segments | Commercial
Total revenues	345	269
Canada | Operating Segments | International
Total revenues	98	93
Europe | Operating Segments | International
Total revenues	143	127
Hardy | Operating Segments | International
Total revenues	$ 105	$ 93