| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Specialty - Surety (Details) - Specialty - Surety [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                                                                                                                                              | $ (64)          | $ (65)          | $ (75)          |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years                                                                                                                                  | 1               | (4)             | (7)             |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Surety                                                                                                                                      | (63)            | (69)            | (82)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | 1               | 0               | (2)             | $ (5)           | $ (3)           | $ 2             | $ (24)          | $ (26)          | $ 9              |  |   |   |   |   |   |   |   | $ (48) |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (1)             | (3)             | (4)             | (9)             | (7)             | (5)             | (41)            | 0               |                  |   |   |   |   |   |   |   | $ (70) |   |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | 1               | (7)             | (9)             | (17)            | (18)            | (11)            | 0               |                 |             |   |   |   |   |   |   | $ (61) |   |   |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (3)             | (10)            | (8)             | (27)            | (1)             | 0               |                 |            |             |   |   |   |   |   | $ (49) |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (5)             | (12)            | (29)            | (5)             | 1               |                 |            |            |             |   |   |   |   | $ (50) |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (18)            | (28)            | (24)            | 2               |                 |            |            |            |             |   |   |   | $ (68) |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (9)             | (6)             | 1               |                 |            |            |            |            |             |   |   | $ (14) |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | (30)            | 1               |                 |            |            |            |            |            |             |   | $ (29) |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                              |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                              | $ 0             |                 |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Surety
+ References
No definition available.
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| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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+ References
No definition available.
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