| **General (Narrative) (Details) - USD ($) $ in Millions**                          |        | 3 Months Ended | 9 Months Ended |         |        |
|------------------------------------------------------------------------------------|--------|----------------|----------------|---------|--------|
||
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Net unrealized gains (losses) on investments                                       |   | $ (159)        | $ 24           | $ (757) | $ 164  |   |   |
| Retained Earnings                                                                  |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      |   |           |           |    |   | $ (50) | $ 0 |
| Accumulated Other Comprehensive (Loss) Income                                      |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      |   |           |           |    |   | (16) | $ 0 |
| ASU 2016-01 | Retained Earnings                                                    |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      | $ 28   |           |           |    |   |   |   |
| ASU 2016-01 | Accumulated Other Comprehensive (Loss) Income                        |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      |   |           |           |    |   | (28) |   |
| ASU 2018-02                                                                        |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Net unrealized gains (losses) on investments                                       | 142    |           |           |    |   |   |   |
| Decrease in unrecognized pension and postretirement benefits                       | 130    |           |           |    |   |   |   |
| ASU 2018-02 | Retained Earnings                                                    |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      | (12)   |           |           |    |   |   |   |
| ASU 2018-02 | Accumulated Other Comprehensive (Loss) Income                        |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Cumulative effect adjustments from changes in accounting guidance, net of tax      | $ 12   |           |           |    |   | $ 12 |   |
| Loews | CNAF Consolidated                                                          |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Noncontrolling interest, ownership percentage by parent                            |   | 89.00%         |           | 89.00%  |   |   |   |
| Common Stock                                                                       |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Realized gains (losses) on equity securities                                       |   | $ (2)          |           | $ (2)   |   |   |   |
| Common Stock | ASU 2016-01                                                         |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Realized gains (losses) on equity securities                                       |   | (1)            |           | 1       |   |   |   |
| Change in fair value of equity securities recognized in other comprehensive income |   |           | 1              |    | 1      |   |   |
| Preferred stock                                                                    |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Realized gains (losses) on equity securities                                       |   | 2              |           | (23)    |   |   |   |
| Preferred stock | ASU 2016-01                                                      |   |           |           |    |   |   |   |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]**   |   |           |           |    |   |   |   |
| Realized gains (losses) on equity securities                                       |   | $ 2            |           | $ (23)  |   |   |   |
| Change in fair value of equity securities recognized in other comprehensive income |   |           | $ 1            |    | $ 7    |   |   |

| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unrealized change in value of total investments, after tax.
+ References
No definition available.
+ Details**Name:**cna_NetUnrealizedGainsLossesOnInvestments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.
+ References
No definition available.
+ Details**Name:**us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 321 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714
+ Details**Name:**us-gaap_EquitySecuritiesFvNiGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of increase (decrease) in obligation for pension and other postretirement benefits. Includes, but is not limited to, defined benefit and defined contribution plans.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585
+ Details**Name:**us-gaap_IncreaseDecreaseInPensionAndPostretirementObligations**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The parent entity's interest in net assets of the subsidiary, expressed as a percentage.
+ References
No definition available.
+ Details**Name:**us-gaap_MinorityInterestOwnershipPercentageByParent**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580
+ Details**Name:**us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_RetainedEarningsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201601Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=cna_AccountingStandardsUpdate201802Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=cna_LoewsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_OwnershipAxis=cna_CNAFConsolidatedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommonStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_PreferredStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |