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v3.8.0.1

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%

X
- Definition Percentage of direct written premiums derived outside of the United States + References No definition available. + Details
Name: | cna_Percentageofforeigndirectwrittenpremiums
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration