Business Segments (Narrative) (Details)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Percentage of written premiums foreign	8.80%	6.90%	7.00%

Percentage of direct written premiums derived outside of the United States

No definition available.

Name:	cna_PercentageOfWrittenPremiumsForeign
Namespace Prefix:	cna_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration