| **Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Changes in Accumulated other comprehensive income (loss) [Roll Forward]**                                                        |           |   |
| Accumulated other comprehensive income (loss), beginning balance                                                                   | $ 831          |   |
| Other comprehensive income (loss), before reclassifications                                                                        | (96)           |   |
| Amounts reclassified from accumulated other comprehensive income (loss)                                                            | 8              |   |
| Other comprehensive income (loss), net of tax                                                                                      | (104)          | 284 |
| Accumulated other comprehensive income (loss), ending balance                                                                      | 727            |   |
| **Other comprehensive income (loss), tax, portion attributable to parent, parenthetical disclosures [Abtract]**                    |           |   |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)                                   | (3)            |   |
| Tax (expense) benefit on other comprehensive income (loss), tax                                                                    | 25             |   |
| Net unrealized gains (losses) on investments with OTTI losses, tax [Member]                                                        |           |   |
| **Other comprehensive income (loss), tax, portion attributable to parent, parenthetical disclosures [Abtract]**                    |           |   |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)                                   |      |   |
| Tax (expense) benefit on other comprehensive income (loss), tax                                                                    | (7)            |   |
| Net unrealized gains (losses) on other investments, tax [Member]                                                                   |           |   |
| **Other comprehensive income (loss), tax, portion attributable to parent, parenthetical disclosures [Abtract]**                    |           |   |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)                                   | (6)            |   |
| Tax (expense) benefit on other comprehensive income (loss), tax                                                                    | 35             |   |
| Pension and postretirement benefits member, tax [Member]                                                                           |           |   |
| **Other comprehensive income (loss), tax, portion attributable to parent, parenthetical disclosures [Abtract]**                    |           |   |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)                                   | 3              |   |
| Tax (expense) benefit on other comprehensive income (loss), tax                                                                    | (3)            |   |
| Cumulative foreign currency translation adjustment, tax [Member]                                                                   |           |   |
| **Other comprehensive income (loss), tax, portion attributable to parent, parenthetical disclosures [Abtract]**                    |           |   |
| Tax (expense) benefit on amounts reclassified from accumulated other comprehensive income (loss)                                   |      |   |
| Tax (expense) benefit on other comprehensive income (loss), tax                                                                    |      |   |
| Net unrealized gains (losses) on investments with OTTI Losses [Member]                                                             |           |   |
| **Changes in Accumulated other comprehensive income (loss) [Roll Forward]**                                                        |           |   |
| Accumulated other comprehensive income (loss), beginning balance                                                                   | 20             |   |
| Other comprehensive income (loss), before reclassifications                                                                        | 14             |   |
| Amounts reclassified from accumulated other comprehensive income (loss)                                                            |      |   |
| Other comprehensive income (loss), net of tax                                                                                      | 14             |   |
| Accumulated other comprehensive income (loss), ending balance                                                                      | 34             |   |
| Net unrealized gains (losses) on other investments [Member]                                                                        |           |   |
| **Changes in Accumulated other comprehensive income (loss) [Roll Forward]**                                                        |           |   |
| Accumulated other comprehensive income (loss), beginning balance                                                                   | 1,371          |   |
| Other comprehensive income (loss), before reclassifications                                                                        | (49)           |   |
| Amounts reclassified from accumulated other comprehensive income (loss)                                                            | 13             |   |
| Other comprehensive income (loss), net of tax                                                                                      | (62)           |   |
| Accumulated other comprehensive income (loss), ending balance                                                                      | 1,309          |   |
| Pension and postretirement benefits [Member]                                                                                       |           |   |
| **Changes in Accumulated other comprehensive income (loss) [Roll Forward]**                                                        |           |   |
| Accumulated other comprehensive income (loss), beginning balance                                                                   | (721)          |   |
| Other comprehensive income (loss), before reclassifications                                                                        |      |   |
| Amounts reclassified from accumulated other comprehensive income (loss)                                                            | (5)            |   |
| Other comprehensive income (loss), net of tax                                                                                      | 5              |   |
| Accumulated other comprehensive income (loss), ending balance                                                                      | (716)          |   |
| Cumulative foreign currency translation adjustment [Member]                                                                        |           |   |
| **Changes in Accumulated other comprehensive income (loss) [Roll Forward]**                                                        |           |   |
| Accumulated other comprehensive income (loss), beginning balance                                                                   | 161            |   |
| Other comprehensive income (loss), before reclassifications                                                                        | (61)           |   |
| Amounts reclassified from accumulated other comprehensive income (loss)                                                            |      |   |
| Other comprehensive income (loss), net of tax                                                                                      | (61)           |   |
| Accumulated other comprehensive income (loss), ending balance                                                                      | $ 100          |   |

| X                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_ChangesInAccumulatedOtherComprehensiveIncomeLossRollForward**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other comprehensive income, before reclassifications, net of tax
+ References
No definition available.
+ Details**Name:**cna_OtherComprehensiveIncomeBeforeReclassificationsNetOfTax**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reclassification from accumulated other comprehensive income, current period, net of tax
+ References
No definition available.
+ Details**Name:**cna_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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
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 Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 14, 17, 26 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: 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=20435746&loc=d3e637-108580Reference 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=20435746&loc=SL7669686-108580Reference 4: 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=20435746&loc=d3e681-108580Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15D -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net of tax amount of other comprehensive income (loss) attributable to the parent entity.
+ References
Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 29 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 38 -Subparagraph c(3) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 30 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 6: 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
Tax effect 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 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569643-111683Reference 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 50 -Paragraph 1A -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=18733093&loc=SL4573702-111684
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParentAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |