| **Expected Credit Losses - Uncollectible Reinsurance and Insurance Receivables (Summary of Outstanding Amount of Voluntary Reinsurance Receivables) (Details) $ in Millions** | Sep. 30, 2020 USD ($) |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------|
| **Financing Receivable, Credit Quality Indicator [Line Items]**                                                                                                               |                  |
| Reinsurance receivables                                                                                                                                                       | $ 3,607               |
| A- to A                                                                                                                                                                       |                  |
| **Financing Receivable, Credit Quality Indicator [Line Items]**                                                                                                               |                  |
| Reinsurance receivables                                                                                                                                                       | 2,729                 |
| B- to B                                                                                                                                                                       |                  |
| **Financing Receivable, Credit Quality Indicator [Line Items]**                                                                                                               |                  |
| Reinsurance receivables                                                                                                                                                       | 874                   |
| Insolvent                                                                                                                                                                     |                  |
| **Financing Receivable, Credit Quality Indicator [Line Items]**                                                                                                               |                  |
| Reinsurance receivables                                                                                                                                                       | $ 4                   |

| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Voluntary Reinsurance Recoverable, Gross
+ References
No definition available.
+ Details**Name:**cna_VoluntaryReinsuranceRecoverableGross**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_CreditRatingAxis=cna_AMinusToAPlusPlusMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_CreditRatingAxis=cna_BMinusToBPlusPlusMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_CreditRatingAxis=cna_InsolventMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |