| **Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ in Millions** | Dec. 31, 2021 USD ($) claim | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 2,582                     |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | 433                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2012                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | 295                         | $ 296                 | $ 297                 | $ 299                 | $ 299                 | $ 307                 | $ 303                 | $ 299                 | $ 289                 | $ 275                 |
| IBNR                                                                                                                                                                                                                       | $ 1                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 46,288                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2013                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 241                       | 241                   | 241                   | 245                   | 245                   | 249                   | 265                   | 265                   | $ 246                 |                  |
| IBNR                                                                                                                                                                                                                       | $ 2                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 39,430                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2014                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 202                       | 201                   | 201                   | 205                   | 205                   | 212                   | 223                   | $ 234                 |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 2                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 33,628                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2015                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 183                       | 181                   | 183                   | 190                   | 190                   | 199                   | $ 201                 |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 3                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 30,427                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2016                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 195                       | 190                   | 186                   | 186                   | 186                   | $ 198                 |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 3                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 30,449                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2017                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 232                       | 221                   | 200                   | 198                   | $ 199                 |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 5                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 30,940                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2018                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 245                       | 227                   | 227                   | $ 229                 |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 8                         |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 34,292                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2019                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 289                       | 266                   | $ 257                 |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 31                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 37,142                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2020                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 303                       | $ 310                 |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 107                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 28,837                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2021                                                                                                                                                                                                                       |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| **Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]**                                                                                                                      |                        |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred claims and allocated claim adjustment expense, net                                                                                                                                                                | $ 397                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| IBNR                                                                                                                                                                                                                       | $ 271                       |                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative Number of Claims | claim                                                                                                                                                                                        | 27,182                      |                  |                  |                  |                  |                  |                  |                  |                  |                  |

| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Net Incurred Claim and Claim Adjustment Expense [Table]
+ References
No definition available.
+ Details**Name:**cna_NetIncurredClaimandClaimAdjustmentExpenseLineItems**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671321-158438Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671317-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredButNotReportedIbnrClaimsLiabilityNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cumulative number of reported claims for short-duration insurance contracts.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671317-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsNumberOfReportedClaims**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_CommercialAutoProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2012Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2013Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2014Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2015Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2016Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2017Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2018Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2019Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2020Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2021Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |