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Quarterly Financial Data (Unaudited) Quarterly Financial Data (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2012
Effect of Fourth Quarter Events [Line Items]
Catastrophe losses, net of reinsurance and including reinstatement premiums	$ 280

X

- Definition

Catastrophe losses, net of reinsurance and including reinstatement premiums related to Storm Sandy

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_EffectOfFourthQuarterEventsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration