Business Segments - Revenues by Line of Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 2,398	$ 2,433	$ 7,094	$ 6,976
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Revenues	940	946	2,779	2,739
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Revenues	915	913	2,640	2,591
Operating Segments | International
Segment Reporting Information [Line Items]
Revenues	244	229	684	658
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Revenues	334	339	1,012	979
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Revenues	(35)	6	(20)	10
Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0	(1)	(1)
Management & Professional Liability | Operating Segments
Segment Reporting Information [Line Items]
Revenues	656	677	1,963	1,954
Surety | Operating Segments
Segment Reporting Information [Line Items]
Revenues	144	139	404	399
Warranty & Alternative Risks | Operating Segments
Segment Reporting Information [Line Items]
Revenues	140	130	412	386
Middle Market | Operating Segments
Segment Reporting Information [Line Items]
Revenues	501	463	1,421	1,298
Small Business | Operating Segments
Segment Reporting Information [Line Items]
Revenues	131	154	357	448
Other Commercial Insurance | Operating Segments
Segment Reporting Information [Line Items]
Revenues	283	296	862	845
Canada | Operating Segments
Segment Reporting Information [Line Items]
Revenues	60	51	164	152
CNA Europe | Operating Segments
Segment Reporting Information [Line Items]
Revenues	87	82	239	241
Hardy | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 97	$ 96	$ 281	$ 265