Fair Value (Narrative) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value Disclosures [Abstract]
Transfers from Level 2 to Level 1	$ 29,000,000		$ 0
Transfers from Level 1 to Level 2	0		$ 0
Other invested assets overseas deposit	$ 42,000,000	$ 39,000,000

Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical experiment, which can be redeemed at net asset value in 90 days or less.

No definition available.

+ Details

Name:	cna_OtherInvestedAssetsOverseasDeposit
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
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