Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues	$ 2,685	$ 2,622	$ 7,990	$ 7,731
Net investment gains (losses)	7	14	20	25
Operating Segments | Specialty
Revenues	1,126	1,066	3,330	3,160
Operating Segments | Commercial
Revenues	959	934	2,839	2,752
Operating Segments | International
Revenues	252	270	776	782
Operating Segments | Life & Group
Revenues	336	332	1,006	996
Operating Segments | Corporate & Other
Revenues	7	7	25	18
Eliminations
Revenues	(2)	(1)	(6)	(2)
Total operating revenues
Revenues	2,678	2,608	7,970	7,706
Management & Professional Liability | Operating Segments | Specialty
Revenues	638	616	1,903	1,867
Surety | Operating Segments | Specialty
Revenues	156	153	446	427
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues	332	297	981	866
Middle Market | Operating Segments | Commercial
Revenues	560	530	1,652	1,555
Small Business | Operating Segments | Commercial
Revenues	115	125	351	364
Other Commercial Insurance | Operating Segments | Commercial
Revenues	284	279	836	833
Canada | Operating Segments | International
Revenues	70	66	204	187
Europe | Operating Segments | International
Revenues	91	94	270	273
Hardy | Operating Segments | International
Revenues	$ 91	$ 110	$ 302	$ 322