| **Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance as of beginning of the period                                                                                                                                                   | $ 553          | $ 385          | $ 437 | $ 454 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 15             | 0              | 42 | (7) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 15             | (1)            | 44 | (5) |
| Purchases                                                                                                                                                                               | 101            | 122            | 255 | 195 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | (77) |
| Settlements                                                                                                                                                                             | (7)            | (29)           | (19) | (45) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 59             | 45 | 77 |
| Transfers out of Level 3                                                                                                                                                                | (16)           | (32)           | (116) | (95) |
| Balance as of end of the period                                                                                                                                                         | 646            | 504            | 646 | 504 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | (1)            | 2 | (4) |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 16             | 1              | 38 | (4) |
| Corporate and other bonds                                                                                                                                                               |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance as of beginning of the period                                                                                                                                                   | 338            | 94             | 222 | 98 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 14             | 0              | 34 | (1) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 14             | 0              | 34 | (2) |
| Purchases                                                                                                                                                                               | 79             | 67             | 211 | 69 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | (5) |
| Settlements                                                                                                                                                                             | (3)            | (3)            | (7) | (7) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 30             | 0 | 35 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | (32) | 0 |
| Balance as of end of the period                                                                                                                                                         | 428            | 188            | 428 | 188 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 14             | 0              | 29 | (2) |
| States, municipalities and political subdivisions                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance as of beginning of the period                                                                                                                                                   | 0              | 1              | 0 | 1 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 0              | 0              | 0 | 0 |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 0              | 0              | 0 | 0 |
| Purchases                                                                                                                                                                               | 0              | 0              | 0 | 0 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | 0 |
| Settlements                                                                                                                                                                             | 0              | (1)            | 0 | (1) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | 0 | 0 |
| Balance as of end of the period                                                                                                                                                         | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 0              | 0              | 0 | 0 |
| Asset-backed                                                                                                                                                                            |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance as of beginning of the period                                                                                                                                                   | 193            | 273            | 197 | 335 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 1              | 0              | 8 | (6) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 1              | (2)            | 8 | (1) |
| Purchases                                                                                                                                                                               | 22             | 55             | 42 | 126 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | (72) |
| Settlements                                                                                                                                                                             | (4)            | (25)           | (12) | (37) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 29             | 45 | 42 |
| Transfers out of Level 3                                                                                                                                                                | (16)           | (32)           | (84) | (95) |
| Balance as of end of the period                                                                                                                                                         | 196            | 298            | 196 | 298 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | (2)            | 0 | (2) |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 2              | 1              | 9 | (2) |
| Equity securities                                                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance as of beginning of the period                                                                                                                                                   | 22             | 17             | 18 | 20 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 0              | 0              | 0 | 0 |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 0              | 1              | 2 | (2) |
| Purchases                                                                                                                                                                               | 0              | 0              | 2 | 0 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | 0 |
| Settlements                                                                                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | 0 | 0 |
| Balance as of end of the period                                                                                                                                                         | 22             | 18             | 22 | 18 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 1              | 2 | (2) |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 0              | 0              | 0 | 0 |
| Reported in Net investment gains (losses)                                                                                                                                               |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | (1)            | 2 | 2 |
| Reported in Net investment gains (losses) | Corporate and other bonds                                                                                                                   |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | 0              | 0 | (1) |
| Reported in Net investment gains (losses) | States, municipalities and political subdivisions                                                                                           |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | 0              | 0 | 0 |
| Reported in Net investment gains (losses) | Asset-backed                                                                                                                                |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | (2)            | 0 | 5 |
| Reported in Net investment gains (losses) | Equity securities                                                                                                                           |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment gains (losses)                                                                                                                                               | $ 0            | $ 1            | $ 2 | $ (2) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss), Included In Earnings And Other Comprehensive Income (Loss)
+ References
No definition available.
+ Details**Name:**cna_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarningsAndOtherComprehensiveIncomeLoss**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Unrealized Gain (Loss) Included in Other Comprehensive Income (Loss)
+ References
No definition available.
+ Details**Name:**cna_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetUnrealizedGainLossIncludedinOtherComprehensiveIncomeLoss**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IncomeStatementLocationAxis=cna_NetRealizedInvestmentGainLossesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |