Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total revenues	$ 2,820	$ 2,685	$ 7,877	$ 7,990
Total operating revenues	2,793	2,678	7,997	7,970
Net investment gains (losses)	27	7	(120)	20
Operating Segments | Specialty
Total revenues	1,177	1,126	3,366	3,330
Operating Segments | Commercial
Total revenues	1,028	959	2,877	2,839
Operating Segments | International
Total revenues	251	252	743	776
Operating Segments | Life & Group
Total revenues	335	336	1,002	1,006
Operating Segments | Corporate & Other
Total revenues	3	7	13	25
Eliminations
Total revenues	(1)	(2)	(4)	(6)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	668	638	1,881	1,903
Surety | Operating Segments | Specialty
Total revenues	153	156	444	446
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	356	332	1,041	981
Middle Market | Operating Segments | Commercial
Total revenues	381	357	1,071	1,065
Construction | Operating Segments | Commercial
Total revenues	298	265	823	754
Small Business | Operating Segments | Commercial
Total revenues	126	124	352	377
Other Commercial | Operating Segments | Commercial
Total revenues	223	213	631	643
Canada | Operating Segments | International
Total revenues	73	70	214	204
Europe | Operating Segments | International
Total revenues	96	91	282	270
Hardy | Operating Segments | International
Total revenues	$ 82	$ 91	$ 247	$ 302