Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 2,403	$ 2,622	$ 2,574	$ 2,535	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 10,134	$ 9,542	$ 9,366
Net realized investment gains (losses)									(52)	93	50
Operating Segments | Specialty
Revenues									4,180	3,625	3,602
Operating Segments | Commercial
Revenues									3,578	3,571	3,529
Operating Segments | International
Revenues									1,059	909	857
Operating Segments | Life & Group
Revenues									1,333	1,323	1,301
Operating Segments | Corporate & Other
Revenues									39	22	28
Eliminations
Revenues									(3)	(1)	(1)
Total operating revenues
Revenues									10,186	9,449	9,316
Management & Professional Liability | Operating Segments | Specialty
Revenues									2,440	2,533	2,562
Surety | Operating Segments | Specialty
Revenues									571	541	529
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues									1,169	551	511
Middle Market | Operating Segments | Commercial
Revenues									2,045	1,965	1,823
Small Business | Operating Segments | Commercial
Revenues									472	480	582
Other Commercial Insurance | Operating Segments | Commercial
Revenues									1,061	1,126	1,124
Canada | Operating Segments | International
Revenues									255	224	203
Europe | Operating Segments | International
Revenues									363	326	314
Hardy | Operating Segments | International
Revenues									$ 441	$ 359	$ 340