Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues	$ 2,622	$ 2,398	$ 7,731	$ 7,094
Net realized investment gains (losses)	14	(24)	25	62
Operating Segments | Specialty
Revenues	1,066	921	3,160	2,710
Operating Segments | Commercial
Revenues	934	924	2,752	2,648
Operating Segments | International
Revenues	270	240	782	667
Operating Segments | Life & Group revenues
Revenues	332	331	996	992
Operating Segments | Corporate & Other revenues
Revenues	7	6	18	16
Eliminations
Revenues	(1)	0	(2)	(1)
Operating segments and eliminations, excluding segment reconciling Items
Revenues	2,608	2,422	7,706	7,032
Management & Professional Liability | Operating Segments | Specialty
Revenues	616	638	1,867	1,896
Surety | Operating Segments | Specialty
Revenues	153	143	427	403
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues	297	140	866	411
Middle Market | Operating Segments | Commercial
Revenues	530	513	1,555	1,450
Small Business | Operating Segments | Commercial
Revenues	125	131	364	353
Other Commercial Insurance | Operating Segments | Commercial
Revenues	279	280	833	845
Canada | Operating Segments | International
Revenues	66	61	187	164
CNA Europe | Operating Segments | International
Revenues	94	87	273	238
CNA Hardy | Operating Segments | International
Revenues	$ 110	$ 92	$ 322	$ 265