| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member]                                                                                                                               |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 142           | $ 121 |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]                                                                                                            |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Expected call date                                                                                                                                                                                              |       | 3.3 |
| Credit spread                                                                                                                                                                                                   | 1.74%           |   |
| Credit spread adjustment                                                                                                                                                                                        |            | 0.02% |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]                                                                                                            |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Expected call date                                                                                                                                                                                              |       | 5.3 |
| Credit spread                                                                                                                                                                                                   | 19.90%          |   |
| Credit spread adjustment                                                                                                                                                                                        |            | 0.48% |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Weighted Average [Member]                                                                                                   |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Expected call date                                                                                                                                                                                              |       | 4.3 |
| Credit spread                                                                                                                                                                                                   | 3.98%           |   |
| Credit spread adjustment                                                                                                                                                                                        |            | 0.17% |
| Fixed maturity securities [Member] | Market Approach Valuation Technique [Member]                                                                                                                               |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Assets, Fair Value Disclosure                                                                                                                                                                                   |            | 72 |
| Fixed maturity securities [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]                                                                                                            |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Private offering price                                                                                                                                                                                          |            | $ 42.39 |
| Fixed maturity securities [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]                                                                                                            |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Private offering price                                                                                                                                                                                          |            | $ 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                                                                                                                                                                                          |            | $ 100.11 |
| Equity securities [Member] | Market Approach Valuation Technique [Member]                                                                                                                                       |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 10              | 34 |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]                                                                                                                    |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Private offering price                                                                                                                                                                                          | $ 360.12        | $ 4.54 |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]                                                                                                                    |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Private offering price                                                                                                                                                                                          | $ 4,267.66      | $ 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                                                                                                                                                                                          | $ 1,147.95      | $ 571.17 |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member]                                                                                                     |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 88            | $ 100 |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]                                                                                  |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Mortality assumption                                                                                                                                                                                            | 70.00%          | 69.00% |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]                                                                                  |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Mortality assumption                                                                                                                                                                                            | 743.00%         | 883.00% |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Weighted Average [Member]                                                                         |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Mortality assumption                                                                                                                                                                                            | 191.60%         | 208.90% |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Rate [Member]                                                                                     |            |   |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |            |   |
| Discount rate risk premium                                                                                                                                                                                      | 9.00%           | 9.00% |

| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Internally developed credit spread
+ References
No definition available.
+ Details**Name:**cna_DebtInstrumentBasisSpreadOnVariableRateInternallyDeveloped**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| 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:pureItemType**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
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ 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=25499696&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage points added to the reference rate to compute the variable rate on the debt instrument.
+ References
No definition available.
+ Details**Name:**us-gaap_DebtInstrumentBasisSpreadOnVariableRate1**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |