| **Claim and Claim Adjustment Expense Reserves (Commerical - Commercial Auto - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions** | 12 Months Ended |         |         |          |        |        |        |        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|---------|---------|----------|--------|--------|--------|--------|
||
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009                                                                              | $ 4,285         | $ 4,366 | $ 4,167 |     |   |   |   |   |   |   |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                                         | 17,965          | 18,070  | 18,249  | $ 18,576 |   |   |   |   |   |   |
| Commercial                                                                                                                                                                 |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                                         | 7,998           | 8,136   |    |     |   |   |   |   |   |   |
| Commercial Auto | Commercial                                                                                                                                               |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 2,016           |    |    |     |   |   |   |   |   |   |
| Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented                                                                    | 395             |    |    |     |   |   |   |   |   |   |
| Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009                                                                              | 6               |    |    |     |   |   |   |   |   |   |
| Liability for unallocated claim adjustment expenses for accident years presented                                                                                           | 11              |    |    |     |   |   |   |   |   |   |
| Total net liability for unpaid claim and claim adjustment expenses                                                                                                         | 412             |    |    |     |   |   |   |   |   |   |
| Commercial Auto | Commercial | 2009                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 277             | 275     | 274     | 274      | $ 272  | $ 261  | $ 233  | $ 191  | $ 130 | $ 73 |
| Commercial Auto | Commercial | 2010                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 287             | 287     | 286     | 281      | 271    | 246    | 203    | 141    | $ 74 |   |
| Commercial Auto | Commercial | 2011                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 289             | 287     | 284     | 274      | 248    | 199    | 145    | $ 79   |   |   |
| Commercial Auto | Commercial | 2012                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 290             | 285     | 282     | 259      | 220    | 160    | $ 78   |   |   |   |
| Commercial Auto | Commercial | 2013                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 234             | 225     | 200     | 168      | 135    | $ 74   |   |   |   |   |
| Commercial Auto | Commercial | 2014                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 187             | 166     | 137     | 102      | $ 64   |   |   |   |   |   |
| Commercial Auto | Commercial | 2015                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 153             | 130     | 96      | $ 52     |   |   |   |   |   |   |
| Commercial Auto | Commercial | 2016                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 126             | 93      | $ 52    |     |   |   |   |   |   |   |
| Commercial Auto | Commercial | 2017                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | 107             | $ 58    |    |     |   |   |   |   |   |   |
| Commercial Auto | Commercial | 2018                                                                                                                                        |            |    |    |     |   |   |   |   |   |   |
| **Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]**                                                               |            |    |    |     |   |   |   |   |   |   |
| Cumulative paid claims and allocated claim adjustment expense, net                                                                                                         | $ 66            |    |    |     |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Table]
+ References
No definition available.
+ Details**Name:**cna_ShortDurationInsuranceContractsNetCumulativePaidClaimandClaimAdjustmentExpensesLineItems**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after reinsurance, of paid 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 (b) -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsCumulativePaidClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid 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 4C -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671315-158438Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of liability for unallocated claim adjustment expense 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 4C -URI http://asc.fasb.org/extlink&oid=99384497&loc=SL65671315-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseAccumulatedUnallocatedClaimAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_CommercialAutoProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2009Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2010Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2011Member**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:** |