General General (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Sep. 30, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Loss on reinsurance transaction	$ 34

Loss on reinsurance agreement due to the difference between market value and book value of the funds withheld assets at the contract inception date.

No definition available.

Name:	cna_Lossonreinsurancetransaction
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration