Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenues	$ 2,493	$ 2,246	$ 4,996	$ 4,647
CNA Specialty [Domain]
Revenues	954	896	1,910	1,841
International - Specialty [Member]
Revenues	59	53	118	110
Professional & Management Liability [Member]
Revenues	687	645	1,387	1,339
Surety [Member]
Revenues	123	121	239	240
Warranty & Alternative Risks [Member]
Revenues	85	77	166	152
CNA Commercial [Domain]
Revenues	1,104	984	2,205	2,072
Commercial Insurance [Member]
Revenues	827	736	1,653	1,571
International - Commercial [Member]
Revenues	90	90	183	181
Small Business [Member]
Revenues	187	158	369	320
Hardy [Domain]
Revenues	77		144
Life & Group Non-Core [Domain]
Revenues	346	360	709	710
Health [Member]
Revenues	287	287	585	578
Life & Annuity [Member]
Revenues	58	57	119	117
Other
Revenues	1	16	5	15
Corporate and Other Non-Core revenues [Member]
Revenues	13	7	30	25
Eliminations [Member]
Revenues	$ (1)	$ (1)	$ (2)	$ (1)