| **Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions** | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 |
|-----------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Gross Unrealized Gains                                                                              | $ 4,558       | $ 5,795       |          |
| Gross Unrealized Losses                                                                             | 112           | 77            |          |
| Allowance for Credit Losses                                                                         | 18            | 40            | $ 0           |
| Estimated Fair Value                                                                                | 44,373        | 44,604        |          |
| **Total fixed maturity securities trading**                                                         |          |          |          |
| Debt securities, amortized cost                                                                     | 39,952        | 38,953        |          |
| Total fixed maturity securities                                                                     | 44,380        | 44,631        |          |
| Corporate and other bonds                                                                           |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 21,444        | 20,792        |          |
| Gross Unrealized Gains                                                                              | 2,755         | 3,578         |          |
| Gross Unrealized Losses                                                                             | 56            | 22            |          |
| Allowance for Credit Losses                                                                         | 11            | 23            | 0             |
| Estimated Fair Value                                                                                | 24,132        | 24,325        |          |
| **Total fixed maturity securities trading**                                                         |          |          |          |
| Total fixed maturity securities                                                                     | 24,852        | 25,234        |          |
| States, municipalities and political subdivisions                                                   |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 10,358        | 9,729         |          |
| Gross Unrealized Gains                                                                              | 1,599         | 1,863         |          |
| Gross Unrealized Losses                                                                             | 14            | 0             |          |
| Allowance for Credit Losses                                                                         | 0             | 0             |          |
| Estimated Fair Value                                                                                | 11,943        | 11,592        |          |
| **Total fixed maturity securities trading**                                                         |          |          |          |
| Total fixed maturity securities                                                                     | 11,943        | 11,592        |          |
| Residential mortgage-backed                                                                         |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 2,893         | 3,442         |          |
| Gross Unrealized Gains                                                                              | 71            | 146           |          |
| Gross Unrealized Losses                                                                             | 8             | 1             |          |
| Allowance for Credit Losses                                                                         | 0             | 0             |          |
| Estimated Fair Value                                                                                | 2,956         | 3,587         |          |
| Commercial mortgage-backed                                                                          |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 1,987         | 1,933         |          |
| Gross Unrealized Gains                                                                              | 63            | 93            |          |
| Gross Unrealized Losses                                                                             | 19            | 42            |          |
| Allowance for Credit Losses                                                                         | 0             | 17            |          |
| Estimated Fair Value                                                                                | 2,031         | 1,967         |          |
| Other asset-backed                                                                                  |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 2,561         | 2,179         |          |
| Gross Unrealized Gains                                                                              | 54            | 81            |          |
| Gross Unrealized Losses                                                                             | 10            | 9             |          |
| Allowance for Credit Losses                                                                         | 7             | 0             |          |
| Estimated Fair Value                                                                                | 2,598         | 2,251         |          |
| Asset-backed                                                                                        |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 7,441         | 7,554         |          |
| Gross Unrealized Gains                                                                              | 188           | 320           |          |
| Gross Unrealized Losses                                                                             | 37            | 52            |          |
| Allowance for Credit Losses                                                                         | 7             | 17            | $ 0           |
| Estimated Fair Value                                                                                | 7,585         | 7,805         |          |
| **Total fixed maturity securities trading**                                                         |          |          |          |
| Total fixed maturity securities                                                                     | 7,585         | 7,805         |          |
| U.S. Treasury and obligations of government-sponsored enterprises                                   |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 132           | 339           |          |
| Gross Unrealized Gains                                                                              | 1             | 2             |          |
| Gross Unrealized Losses                                                                             | 3             | 3             |          |
| Allowance for Credit Losses                                                                         | 0             | 0             |          |
| Estimated Fair Value                                                                                | 130           | 338           |          |
| Foreign government                                                                                  |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 570           | 512           |          |
| Gross Unrealized Gains                                                                              | 15            | 32            |          |
| Gross Unrealized Losses                                                                             | 2             | 0             |          |
| Allowance for Credit Losses                                                                         | 0             | 0             |          |
| Estimated Fair Value                                                                                | 583           | 544           |          |
| Fixed maturity securities                                                                           |          |          |          |
| **Fixed maturity securities available-for-sale:**                                                   |          |          |          |
| Cost or Amortized Cost                                                                              | 39,945        | 38,926        |          |
| Gross Unrealized Gains                                                                              | 4,558         | 5,795         |          |
| Gross Unrealized Losses                                                                             | 112           | 77            |          |
| Allowance for Credit Losses                                                                         | 18            | 40            |          |
| Estimated Fair Value                                                                                | 44,373        | 44,604        |          |
| Total fixed maturity securities trading                                                             |          |          |          |
| **Total fixed maturity securities trading**                                                         |          |          |          |
| Cost or amortized cost, trading securities                                                          | 7             | 27            |          |
| Estimated fair value, trading securities                                                            | $ 7           | $ 27          |          |

| X                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Securities, Amortized Cost
+ References
No definition available.
+ Details**Name:**cna_DebtSecuritiesAmortizedCost**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**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
No definition available.
+ Details**Name:**us-gaap_DebtSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454
+ Details**Name:**us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost excluding accrued interest, before 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).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454
+ Details**Name:**us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454
+ Details**Name:**us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_DebtSecuritiesUnrealizedGainLossAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=124260329&loc=d3e26610-111562
+ Details**Name:**us-gaap_TradingSecuritiesDebt**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amortized cost of investments in debt securities classified as trading.
+ References
No definition available.
+ Details**Name:**us-gaap_TradingSecuritiesDebtAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_TradingSecuritiesUnrealizedHoldingGainLossAbstract**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_ResidentialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember**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_USTreasuryAndGovernmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |