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Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair value of cash collateral by the Company	$ 1	$ 1
Fair value of cash collateral received from counterparties
Derivative transactions entered into	1,581	1,073
Derivative transactions terminated	$ 1,543	$ 1,076

X

- Definition

Derivative transactions entered into

+ References

No definition available.

+ Details

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

X

- Definition

Derivative transactions terminated

+ References

No definition available.

+ Details

Name:	cna_DerivativeTransactionsTerminated
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

X

- Definition

The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959

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Balance Type:	debit
Period Type:	instant