Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Revenues	$ 2,352	$ 2,463
Specialty [Domain]
Revenues	917	915
Management & Professional Liability [Member]
Revenues	697	704
Surety [Member]
Revenues	120	122
Warranty & Alternative Risks [Member]
Revenues	100	89
Commercial [Domain]
Revenues	895	946
Middle Markets [Member]
Revenues	409	403
Small Business [Member]
Revenues	165	184
Other Commercial Insurance [Member]
Revenues	321	359
International [Domain]
Revenues	206	259
Canada [Member]
Revenues	55	71
CNA Europe [Member]
Revenues	77	88
Hardy [Member]
Revenues	74	100
Life & Group Non-Core [Domain]
Revenues	327	331
Corporate and Other [Domain]
Revenues	8	13
Intersegment Eliminations [Domain]
Revenues	$ (1)	$ (1)