| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Specialty - Other Professional and Management Liability (Details) - Specialty - Other Professional and Management Liability [Member] - USD ($) $ in Millions** | 12 Months Ended | 24 Months Ended | 36 Months Ended | 48 Months Ended | 60 Months Ended | 72 Months Ended | 84 Months Ended | 96 Months Ended | 108 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | $ (134)         | $ 7             | $ (52)          |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years                                                                                                                                                                                                            | 4               | (7)             | (35)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability                                                                                                                                                                 | (130)           | 0               | (87)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | 0               | (19)            | (23)            | $ (20)          | $ (10)          | $ (20)          | $ (52)          | $ (11)          | $ 13             |   |   |   |   |   |   |   | $ (142) |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (18)            | (5)             | (30)            | (35)            | (9)             | (30)            | 21              | $ 17            |             |   |   |   |   |   |   | $ (89) |   |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | 8               | (5)             | 9               | (7)             | (5)             | 30              | $ 44            |            |             |   |   |   |   |   | $ 74 |   |   |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (13)            | (10)            | (2)             | (2)             | 23              | $ 2             |            |            |             |   |   |   |   | $ (2) |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (34)            | (4)             | 15              | 29              | $ 28            |            |            |            |             |   |   |   | $ 34 |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (37)            | (6)             | (18)            | $ (13)          |            |            |            |            |             |   |   | $ (74) |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (39)            | 38              | $ (3)           |            |            |            |            |            |             |   | $ (4) |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | (5)             | $ 18            |            |            |            |            |            |            |             | $ 13 |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                                                                    |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                                                                        | $ 4             |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development prior years
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPriorYears**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability
+ References
No definition available.
+ Details**Name:**cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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_ClaimsDevelopmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                        |
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| X                                                                                                                                                                                                             |
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