| **Consolidated Balance Sheet Parentheticals (Parentheticals) (USD $)In Millions, except Share data, unless otherwise specified** | Mar. 31, 2015                                                                       | Dec. 31, 2014                                                                       |
|----------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------|
| **Statement of Financial Position [Abstract]**                                                                                   |                                                                                |                                                                                |
| Fixed maturities securities at amortized cost                                                                                    | $ 36,875cna_MarketableSecuritiesFixedMaturitiesAtAmortizedCost                      | $ 37,335cna_MarketableSecuritiesFixedMaturitiesAtAmortizedCost                      |
| Equity securities at cost                                                                                                        | 213us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis                       | 210us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis                       |
| Allowance for uncollectible reinsurance receivables                                                                              | 48cna_AllowanceForUncollectibleReinsuranceReceivables                               | 48cna_AllowanceForUncollectibleReinsuranceReceivables                               |
| Allowance for uncollectible insurance receivables                                                                                | 59cna_AllowanceForUncollectibleInsuranceReceivables                                 | 61cna_AllowanceForUncollectibleInsuranceReceivables                                 |
| Accumulated depreciation on property and equipment                                                                               | 364us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment | 364us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment |
| Due to Related Parties                                                                                                           | $ 6us-gaap_DueToRelatedPartiesCurrentAndNoncurrent                                  | $ 153us-gaap_DueToRelatedPartiesCurrentAndNoncurrent                                |
| Common stock, par value                                                                                                          | $ 2.50us-gaap_CommonStockParOrStatedValuePerShare                                   | $ 2.50us-gaap_CommonStockParOrStatedValuePerShare                                   |
| Common stock, shares authorized                                                                                                  | 500,000,000us-gaap_CommonStockSharesAuthorized                                      | 500,000,000us-gaap_CommonStockSharesAuthorized                                      |
| Common stock, shares issued                                                                                                      | 273,040,243us-gaap_CommonStockSharesIssued                                          | 273,040,243us-gaap_CommonStockSharesIssued                                          |
| Common stock, shares outstanding                                                                                                 | 270,241,545us-gaap_CommonStockSharesOutstanding                                     | 269,980,202us-gaap_CommonStockSharesOutstanding                                     |
| Treasury stock, shares                                                                                                           | 2,798,698us-gaap_TreasuryStockShares                                                | 3,060,041us-gaap_TreasuryStockShares                                                |

| X                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Allowance for uncollectible insurance receivables
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Allowance for uncollectible reinsurance receivables
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Marketable securities, fixed maturities, at amortized cost
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face amount or stated value per share of common stock.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The maximum number of common shares permitted to be issued by an entity's charter and bylaws.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of obligations due all related parties.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.
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