| **Claim and Claim Adjustment Expense Reserves (Life & Group Policyholder Reserves) (Details) - USD ($) $ in Millions** | 3 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Abstract]**                                                          |           |   |
| Liability for unpaid claims and claims adjustment expense, structured settlement obligations, period increase (decrease)   | $ 46           |   |
| Liability for unpaid claims and claims adjustment expense, long term care policies, period increase (decrease)             | (37)           |   |
| Liability for unpaid claims and claims adjustment expense, period increase (decrease)                                      |           | $ (56) |
| Premium deficiency and future policy benefit                                                                               | $ 74           | $ 216 |

| 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 |
| X                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense (income) recognized due to changes in the accrued obligation to policyholders that relates to insured events.
+ References
No definition available.
+ Details**Name:**us-gaap_LiabilityForFuturePolicyBenefitsPeriodExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of increase (decrease) in the liability for unpaid claims and claims adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpensePeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**duration |