| **Debt (Schedule of debt instruments) (Details) (USD $)In Millions, unless otherwise specified** | Dec. 31, 2014                                                                                                            | Dec. 31, 2013                                                                                                            |
|--------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------|
| Long term debt                                                                                   | $ 2,559us-gaap_LongTermDebtNoncurrent                                                                                    | $ 2,011us-gaap_LongTermDebtNoncurrent                                                                                    |
| Total debt                                                                                       | 2,559us-gaap_LongTermDebt                                                                                                |                                                                                                                     |
| Debt, Long-term and Short-term, Combined Amount                                                  |                                                                                                                     | 2,560us-gaap_DebtLongtermAndShorttermCombinedAmount                                                                      |
| Senior notes 6.500%, face amount of $350, due August 15, 2016                                    |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 349us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes6500FaceAmountOf350DueAugust152016Member | 349us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes6500FaceAmountOf350DueAugust152016Member |
| Senior notes 6.950%, face amount $150, due January 15, 2018                                      |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 150us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes6950FaceAmount150DueJanuary152018Member  | 149us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes6950FaceAmount150DueJanuary152018Member  |
| Senior notes 7.350%, face amount $350, due November 15, 2019                                     |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 348us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes7350FaceAmount350DueNovember152019Member | 348us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes7350FaceAmount350DueNovember152019Member |
| Senior notes 5.875%, face amount $500, due August 15, 2020                                       |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 497us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes5875FaceAmount500DueAugust152020Member   | 497us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes5875FaceAmount500DueAugust152020Member   |
| Senior notes 5.750%, face amount $400, due August 15, 2021                                       |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 397us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes5750FaceAmount400DueAugust152021Member   | 397us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes5750FaceAmount400DueAugust152021Member   |
| Senior Notes, 3.950%, face amount of $550, due May 15, 2024                                      |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 547us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= cna_SeniorNotes3.950faceamountof550dueMay152024Member   |                                                                                                                     |
| Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023                            |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 241us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= us-gaap_SubordinatedDebtMember                          | 241us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= us-gaap_SubordinatedDebtMember                          |
| Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036             |                                                                                                                     |                                                                                                                     |
| Long term debt                                                                                   | 30us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= us-gaap_JuniorSubordinatedDebtMember                     | 30us-gaap_LongTermDebtNoncurrent/ us-gaap_LongtermDebtTypeAxis= us-gaap_JuniorSubordinatedDebtMember                     |
| Senior notes 5.850%, face amount $549, due December 15, 2014                                     |                                                                                                                     |                                                                                                                     |
| Short term debt                                                                                  |                                                                                                                     | $ 549us-gaap_ShortTermBorrowings/ us-gaap_ShortTermDebtTypeAxis= cna_SeniorNotes5850FaceAmount549DueDecember152014Member |

| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the aggregate of total long-term debt, including current maturities and short-term debt.
+ References
No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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| - Definition
Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.
+ References
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