| **Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - 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, Surety                                                                                             | $ (84)          | $ (63)          | $ (69)          |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Surety                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (82)            | (65)            | (65)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2008                                                                                                                              | 1               | 2               | (4)             |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                              | (3)             | 0               | 0               |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Surety                                                                                             | (84)            | (63)            | (69)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Surety | 2008                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | 0               | (1)             | (3)             | $ (4)           | $ (9)           | $ (7)           | $ (5)           | $ (41)          | $ 0              |   |   |   |   |   |   |   | $ (70) |
| Surety | 2009                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | 0               | 1               | (7)             | (9)             | (17)            | (18)            | (11)            | $ 0             |             |   |   |   |   |   |   | $ (61) |   |
| Surety | 2010                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (4)             | (3)             | (10)            | (8)             | (27)            | (1)             | $ 0             |            |             |   |   |   |   |   | $ (53) |   |   |
| Surety | 2011                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (4)             | (5)             | (12)            | (29)            | (5)             | $ 1             |            |            |             |   |   |   |   | $ (54) |   |   |   |
| Surety | 2012                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (7)             | (18)            | (28)            | (24)            | $ 2             |            |            |            |             |   |   |   | $ (75) |   |   |   |   |
| Surety | 2013                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (15)            | (9)             | (6)             | $ 1             |            |            |            |            |             |   |   | $ (29) |   |   |   |   |   |
| Surety | 2014                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (25)            | (30)            | $ 1             |            |            |            |            |            |             |   | $ (54) |   |   |   |   |   |   |
| Surety | 2015                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | (27)            | 0               |            |            |            |            |            |            |             | $ (27) |   |   |   |   |   |   |   |
| Surety | 2016                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                         | $ 0             | $ 0             |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Surety
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety**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
Unallocated Claim Adjustment Expense Development
+ References
No definition available.
+ Details**Name:**cna_UnallocatedClaimAdjustmentExpenseDevelopment**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 |
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