| **Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Life & Group Policyholder Reserves) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Abstract]**                                                                                             |           |           |   |
| Liability for unpaid claims and claims adjustment expense, long term care policies, period increase (decrease)                                                | $ (25)         | $ (40)         |   |
| Liability for unpaid claims and claims adjustment expense structured settlement obligations period increase (decrease)                                        | (5)            | 2              |   |
| Liability for future policy benefits, gross premium valuation, pretax margin                                                                                  | $ 125          | $ 72           | $ 125 |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability for Future Policy Benefits, Gross Premium Valuation, Pretax Margin
+ References
No definition available.
+ Details**Name:**cna_LiabilityForFuturePolicyBenefitsGrossPremiumValuationPretaxMargin**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability for Unpaid Claims and Claims Adjustment Expense, Long Term Care Policies, Period Increase (Decrease)
+ References
No definition available.
+ Details**Name:**cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLongTermCarePoliciesPeriodIncreaseDecrease**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability For Unpaid Claims And Claims Adjustment Expense Structured Settlement Obligations Period Increase (Decrease)
+ References
No definition available.
+ Details**Name:**cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseStructuredSettlementObligationsPeriodIncreaseDecrease**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |