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Fair Value (Narrative) (Details) - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value Disclosures [Abstract] |  |
Transfers from Level 1 to Level 2 | $ 10,000,000 | $ 0
Transfers from Level 2 to Level 1 | 0 | 0
Other invested assets overseas deposit | $ 39,000,000 | $ 31,000,000

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- Definition 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. + References No definition available. + Details
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Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration