| **General (Narrative) (Details) - USD ($) $ in Millions**                             | Sep. 30, 2016 | Dec. 31, 2015 |
|---------------------------------------------------------------------------------------|---------------|---------------|
| **Accounting Policies [Abstract]**                                                    |          |          |
| Cumulative Effect of New Accounting Principle in Period of Adoption on Other Assets   |          | $ (2)         |
| Cumulative Effect of New Accounting Principle in Period of Adoption on Long Term Debt |          | (2)           |
| Other Invested Assets Overseas Deposit                                                | $ 30          | $ 27          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of decrease (increase) to long term debt, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.
+ References
No definition available.
+ Details**Name:**cna_CumulativeEffectofNewAccountingPrincipleinPeriodofAdoptiononLongTermDebt**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of increase (decrease) to other assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.
+ References
No definition available.
+ Details**Name:**cna_CumulativeEffectofNewAccountingPrincipleinPeriodofAdoptiononOtherAssets**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical expedient, which can be redeemed at net asset value in 90 days or less
+ References
No definition available.
+ Details**Name:**cna_OtherInvestedAssetsOverseasDeposit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_AccountingPoliciesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |