Business Segments (Revenues by line of business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Total revenues	$ 2,926	$ 3,029	$ 5,811	$ 5,895
Total operating revenues	2,985	2,991	5,881	5,800
Net investment gains (losses)	(59)	38	(70)	95
Operating Segments | Specialty
Total revenues	1,286	1,255	2,544	2,445
Operating Segments | Commercial
Total revenues	1,091	1,060	2,121	2,068
Operating Segments | International
Total revenues	284	281	561	547
Operating Segments | Life & Group
Total revenues	320	390	651	730
Operating Segments | Corporate & Other
Total revenues	5	6	7	13
Eliminations
Total revenues	(1)	(1)	(3)	(3)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	680	693	1,353	1,360
Surety | Operating Segments | Specialty
Total revenues	162	155	310	297
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	444	407	881	788
Middle Market | Operating Segments | Commercial
Total revenues	374	381	736	756
Construction | Operating Segments | Commercial
Total revenues	350	324	674	633
Small Business | Operating Segments | Commercial
Total revenues	143	143	281	269
Other Commercial | Operating Segments | Commercial
Total revenues	224	212	430	410
Canada | Operating Segments | International
Total revenues	91	87	179	166
Europe | Operating Segments | International
Total revenues	118	115	238	226
Hardy | Operating Segments | International
Total revenues	$ 75	$ 79	$ 144	$ 155