| **Investments (Net realized investment gains (losses)) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                      |
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| Net realized investment gains (losses)                                                                            | $ 10us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet                                                                                      | $ 46us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet |
| Fixed maturity securities [Member]                                                                                |                                                                                                                                                |   |
| Gross realized gains                                                                                              | 33us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember        | 53us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember |
| Gross realized losses                                                                                             | (21)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember     | (15)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember |
| Net realized investment gains (losses)                                                                            | 12us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember  | 38us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember |
| Equity securities [Member]                                                                                        |                                                                                                                                                |   |
| Gross realized gains                                                                                              | 1us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember              | 5us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember |
| Gross realized losses                                                                                             | (1)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember           | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember |
| Net realized investment gains (losses)                                                                            | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember        | 5us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember |
| Derivative [Member]                                                                                               |                                                                                                                                                |   |
| Net realized investment gains (losses)                                                                            | (1)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_DerivativeMember            | 0us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_DerivativeMember |
| Short term investments and other [Member]                                                                         |                                                                                                                                                |   |
| Net realized investment gains (losses)                                                                            | $ (1)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cna_ShorttermInvestandotherMember | $ 3us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cna_ShorttermInvestandotherMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
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| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossRealizedGains**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                         |
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| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cna_ShorttermInvestandotherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
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| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DerivativeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
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| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                           |
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| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |