Business Segments (Narrative) (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Percentage of direct foreign written premiums	7.70%	7.90%	8.00%

Percentage of direct written premiums derived outside of the United States

No definition available.

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