Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues	$ 2,440	$ 2,450	$ 4,903	$ 4,905
CNA Specialty [Domain]
Revenues	993	954	1,972	1,910
International - Specialty [Member]
Revenues	62	59	126	118
Management & Professional Liability [Member]
Revenues	710	687	1,414	1,387
Surety [Member]
Revenues	125	123	247	239
Warranty & Alternative Risks [Member]
Revenues	96	85	185	166
CNA Commercial [Domain]
Revenues	1,027	1,104	2,068	2,205
Commercial Insurance [Member]
Revenues	755	827	1,517	1,653
International - Commercial [Member]
Revenues	91	90	186	183
Small Business [Member]
Revenues	181	187	365	369
Hardy [Domain]
Revenues	91	77	191	144
Life and Group Non-Core [Member]
Revenues	316	303	647	618
Corporate and Other Non-Core [Member]
Revenues	15	13	28	30
Eliminations [Member]
Revenues	$ (2)	$ (1)	$ (3)	$ (2)