| **Fair Value - Assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Millions** | Jun. 30, 2017 | Dec. 31, 2016 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | $ 43,114      | $ 42,406      |
| Liabilities                                                                                                           | 1             | (3)           |
| Corporate and other bonds                                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 19,406        | 18,970        |
| States, municipalities and political subdivisions                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 13,826        | 13,240        |
| Residential mortgage-backed                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 4,921         | 5,073         |
| Commercial mortgage-backed                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1,952         | 2,040         |
| Other asset-backed                                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1,061         | 1,025         |
| Total asset-backed                                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 7,934         | 8,138         |
| U.S. Treasury and obligations of government-sponsored enterprises                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 115           | 93            |
| Foreign government                                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 449           | 445           |
| Redeemable preferred stock                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 19            | 19            |
| Fixed maturity securities                                                                                             |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 41,749        | 40,905        |
| Equity securities                                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 118           | 110           |
| Other invested assets                                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 5             | 5             |
| Short term investments                                                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1,241         | 1,328         |
| Life settlement contracts, included in Other assets                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1             | 58            |
| Other liabilities                                                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Liabilities                                                                                                           | 1             | (3)           |
| Level 1                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 493           | 678           |
| Liabilities                                                                                                           | 0             | 0             |
| Level 1 | Corporate and other bonds                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | States, municipalities and political subdivisions                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Residential mortgage-backed                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Commercial mortgage-backed                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Other asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Total asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | U.S. Treasury and obligations of government-sponsored enterprises                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 115           | 93            |
| Level 1 | Foreign government                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Redeemable preferred stock                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 19            | 19            |
| Level 1 | Fixed maturity securities                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 134           | 112           |
| Level 1 | Equity securities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 99            | 91            |
| Level 1 | Other invested assets                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Short term investments                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 260           | 475           |
| Level 1 | Life settlement contracts, included in Other assets                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 1 | Other liabilities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Liabilities                                                                                                           | 0             | 0             |
| Level 2                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 42,282        | 41,321        |
| Liabilities                                                                                                           | 1             | (3)           |
| Level 2 | Corporate and other bonds                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 19,306        | 18,840        |
| Level 2 | States, municipalities and political subdivisions                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 13,825        | 13,239        |
| Level 2 | Residential mortgage-backed                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 4,798         | 4,944         |
| Level 2 | Commercial mortgage-backed                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1,939         | 2,027         |
| Level 2 | Other asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 979           | 968           |
| Level 2 | Total asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 7,716         | 7,939         |
| Level 2 | U.S. Treasury and obligations of government-sponsored enterprises                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 2 | Foreign government                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 449           | 445           |
| Level 2 | Redeemable preferred stock                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 2 | Fixed maturity securities                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 41,296        | 40,463        |
| Level 2 | Equity securities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 2 | Other invested assets                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 5             | 5             |
| Level 2 | Short term investments                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 981           | 853           |
| Level 2 | Life settlement contracts, included in Other assets                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 2 | Other liabilities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Liabilities                                                                                                           | 1             | (3)           |
| Level 3                                                                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 339           | 407           |
| Liabilities                                                                                                           | 0             | 0             |
| Level 3 | Corporate and other bonds                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 100           | 130           |
| Level 3 | States, municipalities and political subdivisions                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1             | 1             |
| Level 3 | Residential mortgage-backed                                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 123           | 129           |
| Level 3 | Commercial mortgage-backed                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 13            | 13            |
| Level 3 | Other asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 82            | 57            |
| Level 3 | Total asset-backed                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 218           | 199           |
| Level 3 | U.S. Treasury and obligations of government-sponsored enterprises                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 3 | Foreign government                                                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 3 | Redeemable preferred stock                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 3 | Fixed maturity securities                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 319           | 330           |
| Level 3 | Equity securities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 19            | 19            |
| Level 3 | Other invested assets                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 3 | Short term investments                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 0             | 0             |
| Level 3 | Life settlement contracts, included in Other assets                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Assets                                                                                                                | 1             | 58            |
| Level 3 | Other liabilities                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Liabilities                                                                                                           | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
No definition available.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.
+ References
No definition available.
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