Business Segments - Revenues by Line of Business (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 2,330	$ 2,195
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Revenues	918	865
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Revenues	849	802
Operating Segments | International
Segment Reporting Information [Line Items]
Revenues	215	215
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Revenues	341	315
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Revenues	7	(2)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	0	0
Specialty | Operating Segments
Segment Reporting Information [Line Items]
Revenues	661	618
Surety | Operating Segments
Segment Reporting Information [Line Items]
Revenues	123	127
Warranty & Alternative Risks | Operating Segments
Segment Reporting Information [Line Items]
Revenues	134	120
Middle Market | Operating Segments
Segment Reporting Information [Line Items]
Revenues	458	401
Small Business | Operating Segments
Segment Reporting Information [Line Items]
Revenues	97	143
Other Commercial Insurance | Operating Segments
Segment Reporting Information [Line Items]
Revenues	294	258
Canada | Operating Segments
Segment Reporting Information [Line Items]
Revenues	51	50
CNA Europe | Operating Segments
Segment Reporting Information [Line Items]
Revenues	73	78
Hardy | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 91	$ 87