| **Investments (Net change in unrealized gains (losses) on investments) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Net change in unrealized gains (losses) on investments                                                                            | $ 1,517us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss                                                                                     | $ (2,556)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss | $ 1,875us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss |
| Fixed maturity securities [Member]                                                                                                |                                                                                                                                                              |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | 1,511us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember | (2,541)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember | 1,871us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember |
| Equity securities [Member]                                                                                                        |                                                                                                                                                              |   |   |
| Net change in unrealized gains (losses) on investments                                                                            | 6us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember          | (15)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember | 5us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember |
| Other [Member]                                                                                                                    |                                                                                                                                                              |   |   |
| Net change in unrealized gains (losses) on investments                                                                            |                                                                                                                                                              |   | $ (1)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= cna_OtherInvestedAssetsMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.
+ 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 (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=cna_OtherInvestedAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |