Quarterly Financial Data (Unaudited) (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Dec. 31, 2011
Loss recognition on unlocking actuarial assumptions related to payout annuity contracts	$ 115

After-tax loss recognition on unlocking actuarial assumptions related to payout annuity contracts

No definition available.

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