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

Percentage owned of outstanding common stock by majority shareholder

No definition available.

Name:	cna_PercentageOwnedOfOutstandingCommonStockByMajorityShareholder
Namespace Prefix:	cna_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant