| **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, long term care policies, period decrease                        | $ 25           | $ 40 |
| Liability for unpaid claims and claims adjustment expense, incurred but not reported (IBNR) claims, amount                 | 107            |   |
| Liability for unpaid claims and claims adjustment expense, claim severity adjustment, period decrease                      | 82             |   |
| 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 |

| 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, Claim Severity Adjustment, Period Increase (Decrease)
+ References
No definition available.
+ Details**Name:**cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimSeverityAdjustmentPeriodIncreaseDecrease**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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
Estimated costs as of the balance sheet date of settling insured claims and costs incurred in the claims settlement process for claims that have not yet been submitted to the insurance company for reimbursement.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.W) -URI https://asc.fasb.org/extlink&oid=27015833&loc=d3e570705-122907
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |