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Business Segments (Narrative) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Percentage of foreign written premiums	9.20%	8.80%	6.90%

X

- Definition

Percentage of direct written premiums derived outside of the United States

+ References

No definition available.

+ Details

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