| **Investments - Net investment income (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------|----------------|----------------|
||
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | $ 490          | $ 517          | $ 1,049 | $ 966 |
| Investment expense                                                        | (15)           | (15)           | (29) | (29) |
| Net investment income                                                     | 475            | 502            | 1,020 | 937 |
| Fixed maturity securities                                                 |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 457            | 449            | 912 | 895 |
| Equity securities                                                         |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 2              | 4              | 3 | 7 |
| Limited partnership investments                                           |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 16             | 46             | 106 | 32 |
| Mortgage loans                                                            |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 8              | 13             | 15 | 22 |
| Short term investments                                                    |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 3              | 1              | 6 | 4 |
| Trading portfolio                                                         |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | 4              | 4              | 6 | 6 |
| Other                                                                     |           |           |   |   |
| **Net Investment Income [Line Items]**                                    |           |           |   |   |
| Gross investment income                                                   | $ 0            | $ 0            | $ 1 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Income Including Realized Gains Losses on Trading Securities, Before Expenses
+ References
No definition available.
+ Details**Name:**cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ 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=63488584&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                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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