| **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** | 3 Months Ended | 12 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 45,221       | $ 43,073        |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member]                                                                                                                               |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 116            | 142             |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]                                                                                                            |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Credit spread                                                                                                                                                                                                   | 2.00%          | 2.00%           |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]                                                                                                            |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Credit spread                                                                                                                                                                                                   | 15.00%         | 20.00%          |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Weighted Average [Member]                                                                                                   |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Credit spread                                                                                                                                                                                                   | 4.00%          | 4.00%           |
| Equity securities [Member] | Market Approach Valuation Technique [Member]                                                                                                                                       |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 2              | 10              |
| Private offering price                                                                                                                                                                                          | $ 4,295.00     |            |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]                                                                                                                    |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Private offering price                                                                                                                                                                                          |      | $ 360.00        |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]                                                                                                                    |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Private offering price                                                                                                                                                                                          |           | $ 4,268.00      |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Weighted Average [Member]                                                                                                           |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Private offering price                                                                                                                                                                                          |      | $ 1,148.00      |
| 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                                                                                                                                                                                   | 87             | 88              |
| 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%         | 70.00%          |
| 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%        | 743.00%         |
| 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                                                                                                                                                                                            | 191.00%        | 192.00%         |
| 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%           |
| Level 1 [Member]                                                                                                                                                                                                |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 1,728          | 1,179           |
| Level 2 [Member]                                                                                                                                                                                                |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 42,205         | 40,591          |
| Level 3 [Member]                                                                                                                                                                                                |           |            |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |           |            |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 1,288        | $ 1,303         |

| 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
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                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Risk that one party to the contract will default expressed as a percentage, used as an input to measure fair value.
+ 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 (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsCounterpartyCreditRisk**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |