| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions** | Sep. 30, 2016 | Dec. 31, 2015 |
|-----------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | $ 43,852      | $ 41,442      |
| Fair value of liabilities, measured on a recurring basis                                                              | 8             | (5)           |
| Corporate and other bonds [Member]                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 19,818        | 17,760        |
| States, municipalities and political subdivisions [Member]                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 13,501        | 13,174        |
| Residential mortgage-backed [Member]                                                                                  |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 5,365         | 5,072         |
| Commercial mortgage-backed [Member]                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 2,144         | 2,197         |
| Other asset-backed [Member]                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 959           | 921           |
| Total asset-backed [Member]                                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 8,468         | 8,190         |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 76            | 67            |
| Foreign Government Debt Securities [Member]                                                                           |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 438           | 346           |
| Redeemable preferred stock [Member]                                                                                   |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 20            | 35            |
| Fixed maturity securities [Member]                                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 42,321        | 39,572        |
| Equity securities [Member]                                                                                            |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 116           | 197           |
| Other invested assets [Member]                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 5             | 17            |
| Short term investments [Member]                                                                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 1,343         | 1,582         |
| Life settlement contracts, included in Other assets [Member]                                                          |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 67            | 74            |
| Embedded Derivative Financial Instruments [Member]                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of liabilities, measured on a recurring basis                                                              | 8             | (5)           |
| Level 1 [Member]                                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 625           | 726           |
| Fair value of liabilities, measured on a recurring basis                                                              | 0             | 0             |
| Level 1 [Member] | Corporate and other bonds [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | States, municipalities and political subdivisions [Member]                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Residential mortgage-backed [Member]                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Commercial mortgage-backed [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Other asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Total asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 76            | 66            |
| Level 1 [Member] | Foreign Government Debt Securities [Member]                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Redeemable preferred stock [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 20            | 35            |
| Level 1 [Member] | Fixed maturity securities [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 96            | 101           |
| Level 1 [Member] | Equity securities [Member]                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 97            | 177           |
| Level 1 [Member] | Other invested assets [Member]                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Short term investments [Member]                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 432           | 448           |
| Level 1 [Member] | Life settlement contracts, included in Other assets [Member]                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 1 [Member] | Embedded Derivative Financial Instruments [Member]                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of liabilities, measured on a recurring basis                                                              | 0             | 0             |
| Level 2 [Member]                                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 42,733        | 40,243        |
| Fair value of liabilities, measured on a recurring basis                                                              | 8             | (5)           |
| Level 2 [Member] | Corporate and other bonds [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 19,557        | 17,592        |
| Level 2 [Member] | States, municipalities and political subdivisions [Member]                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 13,500        | 13,172        |
| Level 2 [Member] | Residential mortgage-backed [Member]                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 5,286         | 4,938         |
| Level 2 [Member] | Commercial mortgage-backed [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 2,120         | 2,175         |
| Level 2 [Member] | Other asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 916           | 868           |
| Level 2 [Member] | Total asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 8,322         | 7,981         |
| Level 2 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 1             |
| Level 2 [Member] | Foreign Government Debt Securities [Member]                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 438           | 346           |
| Level 2 [Member] | Redeemable preferred stock [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 2 [Member] | Fixed maturity securities [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 41,817        | 39,092        |
| Level 2 [Member] | Equity securities [Member]                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 2 [Member] | Other invested assets [Member]                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 5             | 17            |
| Level 2 [Member] | Short term investments [Member]                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 911           | 1,134         |
| Level 2 [Member] | Life settlement contracts, included in Other assets [Member]                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 2 [Member] | Embedded Derivative Financial Instruments [Member]                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of liabilities, measured on a recurring basis                                                              | 8             | (5)           |
| Level 3 [Member]                                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 494           | 473           |
| Fair value of liabilities, measured on a recurring basis                                                              | 0             | 0             |
| Level 3 [Member] | Corporate and other bonds [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 261           | 168           |
| Level 3 [Member] | States, municipalities and political subdivisions [Member]                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 1             | 2             |
| Level 3 [Member] | Residential mortgage-backed [Member]                                                               |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 79            | 134           |
| Level 3 [Member] | Commercial mortgage-backed [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 24            | 22            |
| Level 3 [Member] | Other asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 43            | 53            |
| Level 3 [Member] | Total asset-backed [Member]                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 146           | 209           |
| Level 3 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 3 [Member] | Foreign Government Debt Securities [Member]                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 3 [Member] | Redeemable preferred stock [Member]                                                                |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 3 [Member] | Fixed maturity securities [Member]                                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 408           | 379           |
| Level 3 [Member] | Equity securities [Member]                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 19            | 20            |
| Level 3 [Member] | Other invested assets [Member]                                                                     |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 3 [Member] | Short term investments [Member]                                                                    |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 0             | 0             |
| Level 3 [Member] | Life settlement contracts, included in Other assets [Member]                                       |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of assets, measured on a recurring basis                                                                   | 67            | 74            |
| Level 3 [Member] | Embedded Derivative Financial Instruments [Member]                                                 |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                      |          |          |
| Fair value of liabilities, measured on a recurring basis                                                              | $ 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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