Business Segments - Revenues by Line of Business (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 2,366	$ 2,348	$ 4,696	$ 4,543
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Revenues	921	928	1,839	1,793
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Revenues	876	876	1,725	1,678
Operating Segments | International
Segment Reporting Information [Line Items]
Revenues	225	214	440	429
Operating Segments | Life & Group Non-Core
Segment Reporting Information [Line Items]
Revenues	337	325	678	640
Operating Segments | Corporate & Other Non-Core
Segment Reporting Information [Line Items]
Revenues	8	6	15	4
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1)	(1)	(1)	(1)
Specialty | Operating Segments
Segment Reporting Information [Line Items]
Revenues	646	659	1,307	1,277
Surety | Operating Segments
Segment Reporting Information [Line Items]
Revenues	137	133	260	260
Warranty & Alternative Risks | Operating Segments
Segment Reporting Information [Line Items]
Revenues	138	136	272	256
Middle Market | Operating Segments
Segment Reporting Information [Line Items]
Revenues	462	434	920	835
Small Business | Operating Segments
Segment Reporting Information [Line Items]
Revenues	129	151	226	294
Other Commercial Insurance | Operating Segments
Segment Reporting Information [Line Items]
Revenues	285	291	579	549
Canada | Operating Segments
Segment Reporting Information [Line Items]
Revenues	53	51	104	101
CNA Europe | Operating Segments
Segment Reporting Information [Line Items]
Revenues	79	81	152	159
Hardy | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 93	$ 82	$ 184	$ 169