| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions** | Sep. 30, 2024 | Dec. 31, 2023 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | $ 42,579      | $ 40,425      |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 668           | 683           |
| Short-term and other                                                                                                  | 1,721         | 2,008         |
| Total assets                                                                                                          | 44,968        | 43,116        |
| Other liabilities                                                                                                     | 0             | 1             |
| Total liabilities                                                                                                     | 0             | 1             |
| Corporate and other bonds                                                                                             |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 26,751        | 25,132        |
| States, municipalities and political subdivisions                                                                     |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 7,148         | 7,392         |
| Asset-backed                                                                                                          |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 8,680         | 7,901         |
| Common Stock                                                                                                          |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 183           | 191           |
| Non-redeemable preferred stock                                                                                        |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 485           | 492           |
| Level 1                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 225           | 161           |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 218           | 219           |
| Short-term and other                                                                                                  | 1,691         | 1,976         |
| Total assets                                                                                                          | 2,134         | 2,356         |
| Other liabilities                                                                                                     | 0             | 0             |
| Total liabilities                                                                                                     | 0             | 0             |
| Level 1 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 225           | 161           |
| Level 1 | States, municipalities and political subdivisions                                                           |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 0             | 0             |
| Level 1 | Asset-backed                                                                                                |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 0             | 0             |
| Level 1 | Common Stock                                                                                                |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 171           | 167           |
| Level 1 | Non-redeemable preferred stock                                                                              |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 47            | 52            |
| Level 2                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 40,157        | 38,274        |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 436           | 440           |
| Short-term and other                                                                                                  | 30            | 32            |
| Total assets                                                                                                          | 40,623        | 38,746        |
| Other liabilities                                                                                                     | 0             | 1             |
| Total liabilities                                                                                                     | 0             | 1             |
| Level 2 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 25,289        | 23,926        |
| Level 2 | States, municipalities and political subdivisions                                                           |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 7,103         | 7,348         |
| Level 2 | Asset-backed                                                                                                |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 7,765         | 7,000         |
| Level 2 | Common Stock                                                                                                |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 0             | 0             |
| Level 2 | Non-redeemable preferred stock                                                                              |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 436           | 440           |
| Level 3                                                                                                               |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 2,197         | 1,990         |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 14            | 24            |
| Short-term and other                                                                                                  | 0             | 0             |
| Total assets                                                                                                          | 2,211         | 2,014         |
| Other liabilities                                                                                                     | 0             | 0             |
| Total liabilities                                                                                                     | 0             | 0             |
| Level 3 | Corporate and other bonds                                                                                   |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 1,237         | 1,045         |
| Level 3 | States, municipalities and political subdivisions                                                           |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 45            | 44            |
| Level 3 | Asset-backed                                                                                                |          |          |
| **Fixed maturity securities:**                                                                                        |          |          |
| Fixed maturity securities                                                                                             | 915           | 901           |
| Level 3 | Common Stock                                                                                                |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | 12            | 24            |
| Level 3 | Non-redeemable preferred stock                                                                              |          |          |
| **Equity securities:**                                                                                                |          |          |
| Equity securities                                                                                                     | $ 2           | $ 0           |

| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Short-term Investments and Other
+ References
No definition available.
+ Details**Name:**cna_ShorttermInvestmentsandOther**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of asset recognized for present right to economic benefit.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 80 -Name Accounting Standards Codification -Section 55 -Paragraph 14 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480078/944-80-55-14Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 80 -Name Accounting Standards Codification -Section 55 -Paragraph 9 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480078/944-80-55-9Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-1Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1
+ Details**Name:**us-gaap_DebtSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_DebtSecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483467/210-10-45-1Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1A -Publisher FASB -URI https://asc.fasb.org/1943274/2147482736/825-10-45-1A
+ Details**Name:**us-gaap_EquitySecuritiesFvNi**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial and nonfinancial obligations.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2
+ Details**Name:**us-gaap_LiabilitiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of other liabilities.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2EReference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2
+ Details**Name:**us-gaap_OtherLiabilitiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_ShortTermInvestmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommonStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_PreferredStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |