| **Fair Value - Narrative (Details) - USD ($)**        | 3 Months Ended | 6 Months Ended |             |
|-------------------------------------------------------|----------------|----------------|-------------|
||
| **Fair Value Disclosures [Abstract]**                 |           |           |        |   |   |
| Fair value, assets, level 2 to level 1, transfers     | $ 0            | $ 0            | $ 0         | $ 0 |   |
| Fair value, assets, level 1 to level 2, transfers     | 0              | $ 0            | 0           | $ 0 |   |
| Other invested assets overseas deposit                | 37,000,000     |           | 37,000,000  |   | $ 31,000,000 |
| Life settlement contracts, escrow amount              | 1,000,000      |           | $ 1,000,000 |   |   |
| Life settlement contracts, receivable, payment period |           |           | 3 years     |   |   |
| Life settlement contracts, fair value                 | $ 1,000,000    |           | $ 1,000,000 |   | $ 58,000,000 |

| X                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contracts Not Released From Escrow
+ References
No definition available.
+ Details**Name:**cna_ContractsNotReleasedFromEscrow**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets, Level 1 to Level 2, Transfers
+ References
No definition available.
+ Details**Name:**cna_FairValueAssetsLevel1toLevel2Transfers**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets, Level 2 to Level 1, Transfers
+ References
No definition available.
+ Details**Name:**cna_FairValueAssetsLevel2toLevel1Transfers**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Life Settlement Contracts, Receivable, Payment Period
+ References
No definition available.
+ Details**Name:**cna_LifeSettlementContractsReceivablePaymentPeriod**Namespace Prefix:**cna_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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**Name:**cna_OtherInvestedAssetsOverseasDeposit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FairValueDisclosuresAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value amount of life settlement contracts accounted for under the fair value method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 325 -SubTopic 30 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=73718008&loc=d3e42732-111610
+ Details**Name:**us-gaap_LifeSettlementContractsFairValueMethodCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |