| **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** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | $ 407           | $ 473 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 9               | 5 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 9               | (4) |
| Purchases                                                                                                                                                                               | 126             | 296 |
| Sales                                                                                                                                                                                   | (68)            | (61) |
| Settlements                                                                                                                                                                             | (102)           | (157) |
| Transfers into Level 3                                                                                                                                                                  | 169             | 61 |
| Transfers out of Level 3                                                                                                                                                                | (96)            | (206) |
| Balance, Ending, Assets                                                                                                                                                                 | 454             | 407 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               | (5) |
| Corporate and other bonds                                                                                                                                                               |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 130             | 168 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 0               | 1 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 3               | 1 |
| Purchases                                                                                                                                                                               | 18              | 163 |
| Sales                                                                                                                                                                                   | (5)             | (36) |
| Settlements                                                                                                                                                                             | (54)            | (103) |
| Transfers into Level 3                                                                                                                                                                  | 16              | 0 |
| Transfers out of Level 3                                                                                                                                                                | (10)            | (64) |
| Balance, Ending, Assets                                                                                                                                                                 | 98              | 130 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               | 0 |
| States, municipalities and political subdivisions                                                                                                                                       |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 1               | 2 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 0               | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 0               | 0 |
| Purchases                                                                                                                                                                               | 0               | 0 |
| Sales                                                                                                                                                                                   | 0               | 0 |
| Settlements                                                                                                                                                                             | 0               | (1) |
| Transfers into Level 3                                                                                                                                                                  | 0               | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0               | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 1               | 1 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               | 0 |
| Asset-backed                                                                                                                                                                            |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 199             | 209 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 2               | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 3               | (5) |
| Purchases                                                                                                                                                                               | 107             | 133 |
| Sales                                                                                                                                                                                   | 0               | (25) |
| Settlements                                                                                                                                                                             | (43)            | (32) |
| Transfers into Level 3                                                                                                                                                                  | 153             | 61 |
| Transfers out of Level 3                                                                                                                                                                | (86)            | (142) |
| Balance, Ending, Assets                                                                                                                                                                 | 335             | 199 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               | 0 |
| Equity securities                                                                                                                                                                       |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 19              | 20 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 0               | (1) |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 3               | 0 |
| Purchases                                                                                                                                                                               | 1               | 0 |
| Sales                                                                                                                                                                                   | (3)             | 0 |
| Settlements                                                                                                                                                                             | 0               | 0 |
| Transfers into Level 3                                                                                                                                                                  | 0               | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0               | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 20              | 19 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               | (2) |
| Derivative financial instruments                                                                                                                                                        |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 0               |   |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 1               |   |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 0               |   |
| Purchases                                                                                                                                                                               | 0               |   |
| Sales                                                                                                                                                                                   | (1)             |   |
| Settlements                                                                                                                                                                             | 0               |   |
| Transfers into Level 3                                                                                                                                                                  | 0               |   |
| Transfers out of Level 3                                                                                                                                                                | 0               |   |
| Balance, Ending, Assets                                                                                                                                                                 | 0               | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | 0               |   |
| Life settlement contracts, included in Other assets                                                                                                                                     |            |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |            |   |
| Balance, Beginning, Assets                                                                                                                                                              | 58              | 74 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)                                                           | 6               | 5 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                                      | 0               | 0 |
| Purchases                                                                                                                                                                               | 0               | 0 |
| Sales                                                                                                                                                                                   | (59)            | 0 |
| Settlements                                                                                                                                                                             | (5)             | (21) |
| Transfers into Level 3                                                                                                                                                                  | 0               | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0               | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 0               | 58 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)                                                                                             | $ 0             | $ (3) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66048111&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 the income statement for 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=66048111&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 (loss) for 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=66048111&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66048111&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66048111&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&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_FairValueByAssetClassAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=cna_LifeSettlementContractsIncludedInOtherAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |