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General (Narrative) (Details) | Mar. 31, 2013
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Percentage owned of outstanding common stock by majority shareholder | 90.00%

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- Definition Percentage owned of outstanding common stock by majority shareholder + References No definition available. + Details
Name: | cna_PercentageOwnedOfOutstandingCommonStockByMajorityShareholder
Namespace Prefix: | cna_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant