| **Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions** | 12 Months Ended |
|----------------------------------------------------------------------------------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | $ 7,345         | $ 7,069 | $ 6,988 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 7,312           | 6,988 | 6,924 |
| Net investment income                                                                  | 1,817           | 2,034 | 1,988 |
| Non-insurance warranty revenue                                                         | 1,007           | 390 | 361 |
| Other revenues                                                                         | 50              | 37 | 43 |
| Total operating revenues                                                               | 10,186          | 9,449 | 9,316 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 5,547           | 5,288 | 5,270 |
| Policyholders’ dividends                                                         | 25              | 22 | 13 |
| Amortization of deferred acquisition costs                                             | 1,335           | 1,233 | 1,235 |
| Non-insurance warranty expense                                                         | 923             | 299 | 271 |
| Other insurance related expenses                                                       | 1,039           | 1,098 | 1,122 |
| Other expenses                                                                         | 301             | 292 | 318 |
| Total claims, benefits and expenses                                                    | 9,170           | 8,232 | 8,229 |
| Core income (loss) before income tax                                                   | 1,016           | 1,217 | 1,087 |
| Income tax (expense) benefit on core income (loss)                                     | (171)           | (298) | (263) |
| Core income (loss)                                                                     | 845             | 919 | 824 |
| Net realized investment gains (losses)                                                 | (52)            | 93 | 50 |
| Income tax (expense) benefit on net realized investment gains (losses)                 | 14              | (30) | (15) |
| Net realized investment gains (losses), after tax                                      | (38)            | 63 | 35 |
| Net deferred tax asset remeasurement                                                   | 6               | (83) |   |
| Net income                                                                             | 813             | 899 | 859 |
| Operating Segments | Specialty                                                         |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | 2,744           | 2,731 | 2,738 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 2,732           | 2,712 | 2,743 |
| Net investment income                                                                  | 439             | 522 | 497 |
| Non-insurance warranty revenue                                                         | 1,007           | 390 | 361 |
| Other revenues                                                                         | 2               | 1 | 1 |
| Total operating revenues                                                               | 4,180           | 3,625 | 3,602 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 1,526           | 1,533 | 1,489 |
| Policyholders’ dividends                                                         | 5               | 4 | 4 |
| Amortization of deferred acquisition costs                                             | 599             | 590 | 586 |
| Non-insurance warranty expense                                                         | 923             | 299 | 271 |
| Other insurance related expenses                                                       | 279             | 279 | 295 |
| Other expenses                                                                         | 46              | 43 | 41 |
| Total claims, benefits and expenses                                                    | 3,378           | 2,748 | 2,686 |
| Core income (loss) before income tax                                                   | 802             | 877 | 916 |
| Income tax (expense) benefit on core income (loss)                                     | (173)           | (295) | (310) |
| Core income (loss)                                                                     | 629             | 582 | 606 |
| Operating Segments | Commercial                                                        |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | 3,060           | 2,922 | 2,883 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 3,050           | 2,881 | 2,840 |
| Net investment income                                                                  | 500             | 658 | 657 |
| Non-insurance warranty revenue                                                         | 0               | 0 | 0 |
| Other revenues                                                                         | 28              | 32 | 32 |
| Total operating revenues                                                               | 3,578           | 3,571 | 3,529 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 2,053           | 1,930 | 1,905 |
| Policyholders’ dividends                                                         | 20              | 18 | 9 |
| Amortization of deferred acquisition costs                                             | 505             | 481 | 475 |
| Non-insurance warranty expense                                                         | 0               | 0 | 0 |
| Other insurance related expenses                                                       | 505             | 530 | 566 |
| Other expenses                                                                         | 43              | 57 | 36 |
| Total claims, benefits and expenses                                                    | 3,126           | 3,016 | 2,991 |
| Core income (loss) before income tax                                                   | 452             | 555 | 538 |
| Income tax (expense) benefit on core income (loss)                                     | (95)            | (186) | (183) |
| Core income (loss)                                                                     | 357             | 369 | 355 |
| Operating Segments | International                                                     |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | 1,018           | 881 | 821 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 1,001           | 857 | 806 |
| Net investment income                                                                  | 57              | 52 | 51 |
| Non-insurance warranty revenue                                                         | 0               | 0 | 0 |
| Other revenues                                                                         | 1               | 0 | 0 |
| Total operating revenues                                                               | 1,059           | 909 | 857 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 699             | 575 | 492 |
| Policyholders’ dividends                                                         | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 231             | 162 | 174 |
| Non-insurance warranty expense                                                         | 0               | 0 | 0 |
| Other insurance related expenses                                                       | 135             | 162 | 133 |
| Other expenses                                                                         | 14              | (7) | 24 |
| Total claims, benefits and expenses                                                    | 1,079           | 892 | 823 |
| Core income (loss) before income tax                                                   | (20)            | 17 | 34 |
| Income tax (expense) benefit on core income (loss)                                     | 1               | (9) | (13) |
| Core income (loss)                                                                     | (19)            | 8 | 21 |
| Operating Segments | Life & Group                                                  |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | 524             | 536 | 547 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 530             | 539 | 536 |
| Net investment income                                                                  | 801             | 782 | 767 |
| Non-insurance warranty revenue                                                         | 0               | 0 | 0 |
| Other revenues                                                                         | 2               | 2 | (2) |
| Total operating revenues                                                               | 1,333           | 1,323 | 1,301 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 1,218           | 1,269 | 1,286 |
| Policyholders’ dividends                                                         | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0               | 0 | 0 |
| Non-insurance warranty expense                                                         | 0               | 0 | 0 |
| Other insurance related expenses                                                       | 122             | 129 | 132 |
| Other expenses                                                                         | 7               | 7 | 8 |
| Total claims, benefits and expenses                                                    | 1,347           | 1,405 | 1,426 |
| Core income (loss) before income tax                                                   | (14)            | (82) | (125) |
| Income tax (expense) benefit on core income (loss)                                     | 57              | 132 | 145 |
| Core income (loss)                                                                     | 43              | 50 | 20 |
| Operating Segments | Corporate & Other                                             |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | 0               | 0 | 0 |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | 0               | 0 | 0 |
| Net investment income                                                                  | 20              | 20 | 16 |
| Non-insurance warranty revenue                                                         | 0               | 0 | 0 |
| Other revenues                                                                         | 19              | 2 | 12 |
| Total operating revenues                                                               | 39              | 22 | 28 |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 51              | (19) | 98 |
| Policyholders’ dividends                                                         | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0               | 0 | 0 |
| Non-insurance warranty expense                                                         | 0               | 0 | 0 |
| Other insurance related expenses                                                       | (1)             | (1) | (3) |
| Other expenses                                                                         | 193             | 192 | 209 |
| Total claims, benefits and expenses                                                    | 243             | 172 | 304 |
| Core income (loss) before income tax                                                   | (204)           | (150) | (276) |
| Income tax (expense) benefit on core income (loss)                                     | 39              | 60 | 98 |
| Core income (loss)                                                                     | (165)           | (90) | (178) |
| Eliminations                                                                           |            |   |   |
| **Segment Reporting Information [Line Items]**                                         |            |   |   |
| Net written premiums                                                                   | (1)             | (1) | (1) |
| **Operating revenues**                                                                 |            |   |   |
| Net earned premiums                                                                    | (1)             | (1) | (1) |
| Net investment income                                                                  | 0               | 0 | 0 |
| Non-insurance warranty revenue                                                         | 0               | 0 | 0 |
| Other revenues                                                                         | (2)             | 0 | 0 |
| Total operating revenues                                                               | (3)             | (1) | (1) |
| **Claims, Benefits and Expenses**                                                      |            |   |   |
| Net incurred claims and benefits                                                       | 0               | 0 | 0 |
| Policyholders’ dividends                                                         | 0               | 0 | 0 |
| Amortization of deferred acquisition costs                                             | 0               | 0 | 0 |
| Non-insurance warranty expense                                                         | 0               | 0 | 0 |
| Other insurance related expenses                                                       | (1)             | (1) | (1) |
| Other expenses                                                                         | (2)             | 0 | 0 |
| Total claims, benefits and expenses                                                    | (3)             | (1) | (1) |
| Core income (loss) before income tax                                                   | 0               | 0 | 0 |
| Income tax (expense) benefit on core income (loss)                                     | 0               | 0 | 0 |
| Core income (loss)                                                                     | $ 0             | $ 0 | $ 0 |

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Costs From Contract With Customers
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Deferred Income Tax Expense (Benefit), Remeasurement of Deferred Tax Asset
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Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
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Income Tax Expense (Benefit) on Operating Income (Loss)
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The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.
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Net Operating Income (Loss) Attributable to Parent
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Net realized investment gains (losses)
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Operating Income (Loss) before Income Tax
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Operating Revenues
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Other Insurance Related Expenses
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| - Definition
The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.
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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).
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| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
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Amount after tax of income (loss) from continuing operations attributable to the parent.
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Amount of expense classified as other.
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Amount of revenue and income classified as other.
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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.
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| - Definition
Amount of expense for dividends to policyholders.
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| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.
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| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.
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| - Definition
Amount, including tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value-added and excise.
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| - References
No definition available.
+ Details**Name:**us-gaap_RevenuesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**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_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_LifeAndGroupNonCoreMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidationItemsAxis=us-gaap_IntersegmentEliminationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |