| **Debt (Schedule of debt instruments) (Details) - USD ($)**                          | Dec. 31, 2017   | Dec. 31, 2016   |
|--------------------------------------------------------------------------------------|-----------------|-----------------|
| **Debt Instrument [Line Items]**                                                     |            |            |
| Long term debt                                                                       | $ 2,708,000,000 | $ 2,710,000,000 |
| Total                                                                                | $ 2,858,000,000 | 2,710,000,000   |
| Senior notes 6.500%, face amount of $350, due August 15, 2016                        |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 6.95%           |            |
| Face amount                                                                          | $ 150,000,000   |            |
| Short term debt                                                                      | $ 150,000,000   | 0               |
| Senior notes 6.950%, face amount $150, due January 15, 2018                          |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 6.95%           |            |
| Face amount                                                                          | $ 150,000,000   |            |
| Long term debt                                                                       | $ 0             | 150,000,000     |
| Senior notes 7.350%, face amount $350, due November 15, 2019                         |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 7.35%           |            |
| Face amount                                                                          | $ 350,000,000   |            |
| Long term debt                                                                       | $ 0             | 349,000,000     |
| Senior notes 5.875%, face amount $500, due August 15, 2020                           |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 5.875%          |            |
| Face amount                                                                          | $ 500,000,000   |            |
| Long term debt                                                                       | $ 498,000,000   | 498,000,000     |
| Senior notes 5.750%, face amount $400, due August 15, 2021                           |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 5.75%           |            |
| Face amount                                                                          | $ 400,000,000   |            |
| Long term debt                                                                       | $ 398,000,000   | 398,000,000     |
| Senior Notes, 3.950%, face amount of $550, due May 15, 2024                          |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 3.95%           |            |
| Face amount                                                                          | $ 550,000,000   |            |
| Long term debt                                                                       | $ 547,000,000   | 546,000,000     |
| Senior Notes, 4.500%, face amount of $500, due March 1, 2026                         |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 4.50%           |            |
| Face amount                                                                          | $ 500,000,000   |            |
| Long term debt                                                                       | $ 498,000,000   | 498,000,000     |
| Senior Notes, 3.450%, face amount of $500, due August 15, 2027                       |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 3.45%           |            |
| Face amount                                                                          | $ 500,000,000   |            |
| Long term debt                                                                       | $ 495,000,000   | 0               |
| Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023                |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Stated interest rate                                                                 | 7.25%           |            |
| Face amount                                                                          | $ 243,000,000   |            |
| Long term debt                                                                       | 242,000,000     | 241,000,000     |
| Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036 |            |            |
| **Debt Instrument [Line Items]**                                                     |            |            |
| Face amount                                                                          | 30,000,000      |            |
| Long term debt                                                                       | $ 30,000,000    | $ 30,000,000    |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
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| - Definition
Contractual interest rate for funds borrowed, under the debt agreement.
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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.
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No definition available.
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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.
+ References
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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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