| **Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions**    | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    | $ 143          | $ 962          | $ 400 | $ 442 |
| Transfer to net assets held for sale                                                                |           |           |   | 0 |
| Other comprehensive income (loss), before reclassifications                                         | (118)          | (136)          | (385) | 448 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit | (35)           | 53             | (45) | 117 |
| Other comprehensive income (loss) after tax (expense) benefit                                       | (83)           | (189)          | (340) | 331 |
| Accumulated other comprehensive income (loss), ending balance                                       | 60             | 773            | 60 | 773 |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    | 19             | (33)           | 31 | (68) |
| Tax (expense) benefit on other comprehensive income (loss)                                          | 18             | 87             | 113 | (176) |
| Net unrealized gains (losses) on investments with OTTI losses [Member)                              |           |           |   |   |
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    | 31             | 35             | 36 | 26 |
| Transfer to net assets held for sale                                                                |           |           |   | (5) |
| Other comprehensive income (loss), before reclassifications                                         | 2              | 1              | (3) | 15 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit | 0              | 0              | 0 | 0 |
| Other comprehensive income (loss) after tax (expense) benefit                                       | 2              | 1              | (3) | 15 |
| Accumulated other comprehensive income (loss), ending balance                                       | 33             | 36             | 33 | 36 |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    | 0              | 0              | 0 | 0 |
| Tax (expense) benefit on other comprehensive income (loss)                                          | (1)            | (1)            | 1 | (8) |
| Net unrealized gains (losses) on other investments [Member]                                         |           |           |   |   |
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    | 689            | 1,182          | 942 | 692 |
| Transfer to net assets held for sale                                                                |           |           |   | (17) |
| Other comprehensive income (loss), before reclassifications                                         | (67)           | (59)           | (318) | 462 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit | (31)           | 24             | (29) | 38 |
| Other comprehensive income (loss) after tax (expense) benefit                                       | (36)           | (83)           | (289) | 424 |
| Accumulated other comprehensive income (loss), ending balance                                       | 653            | 1,099          | 653 | 1,099 |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    | 17             | (12)           | 22 | (20) |
| Tax (expense) benefit on other comprehensive income (loss)                                          | 21             | 64             | 140 | (209) |
| Accumulated Net Unrealized Gains Losses on discontinued operations [Member]                         |           |           |   |   |
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    |           | 37             |   | 0 |
| Transfer to net assets held for sale                                                                |           |           |   | 22 |
| Other comprehensive income (loss), before reclassifications                                         |           | (3)            |   | 12 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit |           | 34             |   | 34 |
| Other comprehensive income (loss) after tax (expense) benefit                                       |           | (37)           |   | (22) |
| Accumulated other comprehensive income (loss), ending balance                                       |           | 0              |   | 0 |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    |           | (23)           |   | (23) |
| Tax (expense) benefit on other comprehensive income (loss)                                          |           | 25             |   | 15 |
| Pension and postretirement benefits [Member]                                                        |           |           |   |   |
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    | (585)          | (476)          | (633) | (426) |
| Transfer to net assets held for sale                                                                |           |           |   | 0 |
| Other comprehensive income (loss), before reclassifications                                         | 0              | (2)            | 36 | (2) |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit | (4)            | (5)            | (16) | 45 |
| Other comprehensive income (loss) after tax (expense) benefit                                       | 4              | 3              | 52 | (47) |
| Accumulated other comprehensive income (loss), ending balance                                       | (581)          | (473)          | (581) | (473) |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    | 2              | 2              | 9 | (25) |
| Tax (expense) benefit on other comprehensive income (loss)                                          | (2)            | (1)            | (28) | 26 |
| Cumulative foreign currency translation adjustment [Member]                                         |           |           |   |   |
| **Accumulated Other Comprehensive Income (Loss) [Line Items]**                                      |           |           |   |   |
| Accumulated other comprehensive income (loss), beginning balance                                    | 8              | 184            | 55 | 150 |
| Transfer to net assets held for sale                                                                |           |           |   | 0 |
| Other comprehensive income (loss), before reclassifications                                         | (53)           | (73)           | (100) | (39) |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit | 0              | 0              | 0 | 0 |
| Other comprehensive income (loss) after tax (expense) benefit                                       | (53)           | (73)           | (100) | (39) |
| Accumulated other comprehensive income (loss), ending balance                                       | (45)           | 111            | (45) | 111 |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)    | 0              | 0              | 0 | 0 |
| Tax (expense) benefit on other comprehensive income (loss)                                          | $ 0            | $ 0            | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tax Expense Benefit On Reclassification From Accumulated Other Comprehensive Income Current Period
+ References
No definition available.
+ Details**Name:**cna_TaxExpenseBenefitOnReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriod**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Transfer to net assets held for sale
+ References
No definition available.
+ Details**Name:**cna_Transfertonetassetsheldforsale**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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_AccumulatedOtherComprehensiveIncomeLossLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3
+ Details**Name:**us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of other comprehensive income (loss) attributable to parent entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569643-111683
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569643-111683
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherThanTemporaryImpairmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=cna_AccumulatedNetUnrealizedGainsLossesOnOtherInvestmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=cna_AccumulatedNetUnrealizedGainsLossesondiscontinuedoperationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedTranslationAdjustmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |