| **Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Narrative) (Details) - USD ($) $ in Millions**                                                 | 3 Months Ended | 9 Months Ended |         |        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|---------|--------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |           |    |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                                                                                                   | $ 114          | $ 178          | $ 171   | $ 357  |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                                                   | (17)           | (10)           | (2)     | 4      |   |   |
| Retroactive reinsurance benefit recognized                                                                                                                                       | 17             | 8              | 40      | 30     |   |   |
| Hurricane Ian                                                                                                                                                                    |           |           |    |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |           |    |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                                                                                                   | 87             |           | 87      |   |   |   |
| Hurricane Ida                                                                                                                                                                    |           |           |    |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |           |    |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                                                                                                   |           | 114            |    |   |   |   |
| Corporate & Other                                                                                                                                                            |           |           |    |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |           |    |   |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                                                   | 0              | $ 0            | 64      | $ 40   |   |   |
| Asbestos and Environmental Reserves                                                                                                                                              |           |           |    |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |           |    |   |   |   |
| Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO                                                                                                 |           |           |    |   |   | $ 1,600 |
| Aggregate limit under A&EP Loss Portfolio Transfer                                                                                                                           |           |           |    |   |   | 4,000 |
| A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer |           |           |    |   |   | 1,200 |
| Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer                                                                                                          |           |           |    |   |   | 2,000 |
| Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer                                                                                           |           |           |    |   |   | 215 |
| Total consideration paid to NICO under AEP Loss Portfolio Transfer                                                                                                               |           |           |    |   |   | $ 2,200 |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                                                                                                       | 3,400          |           | 3,400   |   | $ 3,400 |   |
| Deferred reinsurance benefit yet to be recognized                                                                                                                                | 389            |           | 389     |   | $ 429 |   |
| Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer                                                                                | $ 2,300        |           | $ 2,300 |   |   |   |

| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate limit under A&EP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Claim and allocated claim adjustment expense reserves under existing third party reinsurance contracts.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cumulative Amounts Ceded Under AEP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred Reinsurance Benefit Yet To Be Recognized
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_NetReinsuranceReceivablesTransferredToNICOUnderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retroactive Reinsurance Benefit Recognized
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Consideration Paid To NICO Under AEP Loss Portfolio Transfer
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
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| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after deduction of insurance proceeds, of loss recognized in the income statement for an event or transaction that is unusual in nature or infrequent in occurrence, or both.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 930 -SubTopic 715 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=109241964&loc=d3e23713-110274Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 20 -Section 50 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=114868883&loc=SL114871943-224233
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