| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) (USD $)** | Mar. 31, 2013 | Dec. 31, 2012 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Fixed maturity securities [Member] | Discounted cash flow valuation technique [Member]                                                            |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Assets, Fair Value Disclosure                                                                                                                     | $ 78,000,000  | $ 121,000,000 |
| Fixed maturity securities [Member] | Discounted cash flow valuation technique [Member] | Minimum [Member]                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Expected call date                                                                                                                                | 3.0           | 3.3           |
| Credit spread adjustment                                                                                                                          | 0.02%         | 0.02%         |
| Fixed maturity securities [Member] | Discounted cash flow valuation technique [Member] | Maximum [Member]                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Expected call date                                                                                                                                | 4.6           | 5.3           |
| Credit spread adjustment                                                                                                                          | 0.48%         | 0.48%         |
| Fixed maturity securities [Member] | Discounted cash flow valuation technique [Member] | Weighted Average [Member]                                |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Expected call date                                                                                                                                | 4.0           | 4.3           |
| Credit spread adjustment                                                                                                                          | 0.17%         | 0.17%         |
| Fixed maturity securities [Member] | Market approach valuation technique [Member]                                                                 |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Assets, Fair Value Disclosure                                                                                                                     | 97,000,000    | 72,000,000    |
| Fixed maturity securities [Member] | Market approach valuation technique [Member] | Minimum [Member]                                              |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 34.70       | $ 42.39       |
| Fixed maturity securities [Member] | Market approach valuation technique [Member] | Maximum [Member]                                              |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 122.09      | $ 102.32      |
| Fixed maturity securities [Member] | Market approach valuation technique [Member] | Weighted Average [Member]                                     |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 102.97      | $ 100.11      |
| Equity securities [Member] | Market approach valuation technique [Member]                                                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Assets, Fair Value Disclosure                                                                                                                     | 19,000,000    | 34,000,000    |
| Equity securities [Member] | Market approach valuation technique [Member] | Minimum [Member]                                                      |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 33.73       | $ 4.54        |
| Equity securities [Member] | Market approach valuation technique [Member] | Maximum [Member]                                                      |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 3,970.99    | $ 3,842.00    |
| Equity securities [Member] | Market approach valuation technique [Member] | Weighted Average [Member]                                             |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Private offering price                                                                                                                            | $ 1,114.32    | $ 571.17      |
| Life settlement contracts [Member] | Discounted cash flow valuation technique [Member]                                                            |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Assets, Fair Value Disclosure                                                                                                                     | $ 95,000,000  | $ 100,000,000 |
| Life settlement contracts [Member] | Discounted cash flow valuation technique [Member] | Minimum [Member]                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Mortality assumption                                                                                                                              | 69.00%        | 69.00%        |
| Life settlement contracts [Member] | Discounted cash flow valuation technique [Member] | Maximum [Member]                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Mortality assumption                                                                                                                              | 883.00%       | 883.00%       |
| Life settlement contracts [Member] | Discounted cash flow valuation technique [Member] | Weighted Average [Member]                                |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Mortality assumption                                                                                                                              | 209.20%       | 208.90%       |
| Life settlement contracts [Member] | Discounted cash flow valuation technique [Member] | Rate [Member]                                            |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                              |          |          |
| Discount rate risk premium                                                                                                                        | 9.00%         | 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
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                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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:**num:perShareItemType**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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The percentage points added to the reference rate to compute the variable rate on the debt instrument.
+ References
No definition available.
+ Details**Name:**us-gaap_DebtInstrumentBasisSpreadOnVariableRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |