| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Commercial - Auto (Details) - Commercial - Auto [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 |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | $ (42)          | $ (19)          | $ 37            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years                                                                                                                                | (4)             | (3)             | (6)             |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto                                                                                                                           | (46)            | (22)            | 31              |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (2)             | (1)             | (1)             | $ (2)           | $ 3             | $ (5)           | $ (8)           | $ 1             | $ 19             |   |   |   |   |   |   |   | $ 4 |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (2)             | 0               | (7)             | (4)             | 3               | (10)            | (7)             | $ 1             |             |   |   |   |   |   |   | $ (26) |   |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (3)             | (2)             | (4)             | 3               | 4               | 2               | $ (15)          |            |             |   |   |   |   |   | $ (15) |   |   |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (5)             | (5)             | 8               | 4               | 5               | $ 12            |            |            |             |   |   |   |   | $ 19 |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (5)             | (3)             | 14              | 6               | $ 11            |            |            |            |             |   |   |   | $ 23 |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | 4               | 4               | 10              | $ 12            |            |            |            |            |             |   |   | $ 30 |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (16)            | (2)             | $ 17            |            |            |            |            |            |             |   | $ (1) |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | (11)            | $ (10)          |            |            |            |            |            |            |             | $ (21) |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                            | $ (2)           |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto**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
+ 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
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.
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| X                                                                                                                                                      |
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| X                                                                                                                                                                                                             |
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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