| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) - USD ($) $ in Millions** | Jun. 30, 2018 | Dec. 31, 2017 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)**                                                                      |          |          |
| Estimated fair value                                                                                                                                              | $ 41,785      | $ 43,536      |
| Fixed maturity securities | Discounted cash flow                                                                                                                  |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)**                                                                      |          |          |
| Estimated fair value                                                                                                                                              | $ 130         | $ 136         |
| Fixed maturity securities | Discounted cash flow | Minimum                                                                                                        |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)**                                                                      |          |          |
| Credit spread                                                                                                                                                     | 1.00%         | 1.00%         |
| Fixed maturity securities | Discounted cash flow | Maximum                                                                                                        |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)**                                                                      |          |          |
| Credit spread                                                                                                                                                     | 12.00%        | 12.00%        |
| Fixed maturity securities | Discounted cash flow | Weighted Average                                                                                               |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)**                                                                      |          |          |
| Credit spread                                                                                                                                                     | 3.00%         | 3.00%         |

| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
fair value inputs, credit risk, counterparty
+ References
No definition available.
+ Details**Name:**cna_Fairvalueinputscreditriskcounterparty**Namespace Prefix:**cna_**Data Type:**num:percentItemType**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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ValuationTechniqueAxis=us-gaap_IncomeApproachValuationTechniqueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_RangeAxis=srt_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_RangeAxis=srt_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_RangeAxis=srt_WeightedAverageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |