Business Segments (Revenues by line of business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total revenues	$ 2,959	$ 2,820	$ 8,854	$ 7,877
Total operating revenues	2,937	2,793	8,737	7,997
Net investment gains (losses)	22	27	117	(120)
Operating Segments | Specialty
Total revenues	1,247	1,177	3,692	3,366
Operating Segments | Commercial
Total revenues	1,041	1,013	3,109	2,842
Operating Segments | International
Total revenues	285	251	832	743
Operating Segments | Life & Group
Total revenues	362	335	1,092	1,002
Operating Segments | Corporate & Other
Total revenues	4	18	17	48
Eliminations
Total revenues	(2)	(1)	(5)	(4)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	684	668	2,044	1,881
Surety | Operating Segments | Specialty
Total revenues	157	153	454	444
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	406	356	1,194	1,041
Middle Market | Operating Segments | Commercial
Total revenues	363	379	1,119	1,069
Construction | Operating Segments | Commercial
Total revenues	345	298	978	823
Small Business | Operating Segments | Commercial
Total revenues	144	126	413	352
Other Commercial | Operating Segments | Commercial
Total revenues	189	210	599	598
Canada | Operating Segments | International
Total revenues	87	73	253	214
Europe | Operating Segments | International
Total revenues	123	96	349	282
Hardy | Operating Segments | International
Total revenues	$ 75	$ 82	$ 230	$ 247