| **Debt (Schedule of debt instruments) (Details) - USD ($) $ in Millions**            | Dec. 31, 2016 | Dec. 31, 2015 |
|--------------------------------------------------------------------------------------|---------------|---------------|
| Long term debt                                                                       | $ 2,710       | $ 2,210       |
| Total debt                                                                           | 2,710         | 2,560         |
| Senior notes 6.950%, face amount $150, due January 15, 2018                          |          |          |
| Long term debt                                                                       | 150           | 150           |
| Senior notes 7.350%, face amount $350, due November 15, 2019                         |          |          |
| Long term debt                                                                       | 349           | 349           |
| Senior notes 5.875%, face amount $500, due August 15, 2020                           |          |          |
| Long term debt                                                                       | 498           | 497           |
| Senior notes 5.750%, face amount $400, due August 15, 2021                           |          |          |
| Long term debt                                                                       | 398           | 397           |
| Senior Notes, 3.950%, face amount of $550, due May 15, 2024                          |          |          |
| Long term debt                                                                       | 546           | 546           |
| Senior Notes, 4.500%, face amount of $500, due March 1, 2026                         |          |          |
| Long term debt                                                                       | 498           | 0             |
| Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023                |          |          |
| Long term debt                                                                       | 241           | 241           |
| Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036 |          |          |
| Long term debt                                                                       | 30            | 30            |
| Senior notes 6.500%, face amount of $350, due August 15, 2016 [Member]               |          |          |
| Short term debt                                                                      | $ 0           | $ 350         |

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| - Definition
Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
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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.
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