| **Investments (Net investment income) (Details) - USD ($) $ in Millions** | 3 Months Ended |
|---------------------------------------------------------------------------|----------------|
||
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | $ 449          | $ 572 |
| Investment expense                                                        | (14)           | (14) |
| Net investment income                                                     | 435            | 558 |
| Fixed maturity securities [Member]                                        |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | 446            | 443 |
| Total equity securities available-for-sale [Member]                       |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | 3              | 3 |
| Limited partnership investments [Member]                                  |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | (14)           | 114 |
| Mortgage loans [Member]                                                   |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | 9              | 8 |
| Short term investments [Member]                                           |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | 3              | 2 |
| Trading portfolio [Member]                                                |           |   |
| **Net Investment Income [Line Items]**                                    |           |   |
| Gross investment income                                                   | $ 2            | $ 2 |

| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Income Including Realized Gains Losses on Trading Securities, Before Expenses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities and income or expense derived from the amortization of investment related discounts or premiums, respectively.  It also includes realized gains (losses) on trading securities.
+ References
No definition available.
+ Details**Name:**cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expenses related to the generation of investment income.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913
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| 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_ScheduleOfInvestmentIncomeReportedAmountsByCategoryLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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