| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) (USD $)** | Sep. 30, 2012 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|---------------|
| Fixed maturity securities [Member] | Discounted cash flow valuation [Member]                                                                      |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Fair value                                                                                                                                        | $ 98,000,000  |
| Fixed maturity securities [Member] | Discounted cash flow valuation [Member] | Minimum [Member]                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Expected call date                                                                                                                                | 3.6           |
| Fixed maturity securities [Member] | Discounted cash flow valuation [Member] | Maximum [Member]                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Expected call date                                                                                                                                | 5.6           |
| Fixed maturity securities [Member] | Discounted cash flow valuation [Member] | Weighted Average [Member]                                          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Expected call date                                                                                                                                | 4.6           |
| Fixed maturity securities [Member] | Market approach valuation [Member]                                                                           |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Fair value                                                                                                                                        | 61,000,000    |
| Fixed maturity securities [Member] | Market approach valuation [Member] | Minimum [Member]                                                        |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 60.00         |
| Fixed maturity securities [Member] | Market approach valuation [Member] | Maximum [Member]                                                        |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 105.00        |
| Fixed maturity securities [Member] | Market approach valuation [Member] | Weighted Average [Member]                                               |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 101.49        |
| Equity securities [Member] | Market approach valuation [Member]                                                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Fair value                                                                                                                                        | 33,000,000    |
| Equity securities [Member] | Market approach valuation [Member] | Minimum [Member]                                                                |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 0.10          |
| Equity securities [Member] | Market approach valuation [Member] | Maximum [Member]                                                                |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 3,842         |
| Equity securities [Member] | Market approach valuation [Member] | Weighted Average [Member]                                                       |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Private offering price                                                                                                                            | 583.95        |
| Equity securities [Member] | Income approach valuation [Member]                                                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Fair value                                                                                                                                        | 17,000,000    |
| EBITDA Projection                                                                                                                                 | 80,000,000    |
| EBITDA Multiple                                                                                                                                   | 1.82          |
| Life settlement contracts [Member] | Discounted cash flow valuation [Member]                                                                      |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Fair value                                                                                                                                        | $ 113,000,000 |
| Life settlement contracts [Member] | Discounted cash flow valuation [Member] | Minimum [Member]                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Mortality assumption                                                                                                                              | 65.00%        |
| Life settlement contracts [Member] | Discounted cash flow valuation [Member] | Maximum [Member]                                                   |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Mortality assumption                                                                                                                              | 928.00%       |
| Life settlement contracts [Member] | Discounted cash flow valuation [Member] | Weighted Average [Member]                                          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Mortality assumption                                                                                                                              | 185.00%       |
| Life settlement contracts [Member] | Discounted cash flow valuation [Member] | Rate [Member]                                                      |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |
| Discount rate risk premium                                                                                                                        | 9.00%         |

| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Discount rate risk premium
+ References
No definition available.
+ Details**Name:**cna_DiscountRateRiskPremium**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Earnings Before Interest, Tax, Depreciation and Amortization Multiplier
+ References
No definition available.
+ Details**Name:**cna_EbitdaMultiple**Namespace Prefix:**cna_**Data Type:**xbrli:decimalItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Earnings Before Interest, Tax, Depreciation and Amortization Projections for equity securities
+ References
No definition available.
+ Details**Name:**cna_EbitdaProjection**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Expected Call Date in years
+ References
No definition available.
+ Details**Name:**cna_ExpectedCallDate**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortality assumption
+ References
No definition available.
+ Details**Name:**cna_MortalityAssumption**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Private offering price
+ References
No definition available.
+ Details**Name:**cna_PrivateOfferingPrice**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
No definition available.
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |