| **Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance, Beginning, Assets                                                                                                                                                              | $ 2,073        | $ 1,923        | $ 2,014 | $ 1,676 |
| Reported in Net investment gains (losses)                                                                                                                                               | (3)            | (4)            | $ (10) | $ (5) |
| Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]                                |           |           | Net investment losses | Net investment losses |
| Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]     |           |           | Other comprehensive income (loss) | Other comprehensive income (loss) |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 91             | (66)           | $ 54 | $ (57) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 92             | (66)           | 65 | (55) |
| Purchases                                                                                                                                                                               | 121            | 90             | 343 | 381 |
| Sales                                                                                                                                                                                   | (10)           | (2)            | (43) | (4) |
| Settlements                                                                                                                                                                             | (47)           | (32)           | (142) | (66) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 11 | 34 |
| Transfers out of Level 3                                                                                                                                                                | (18)           | (7)            | (37) | (60) |
| Balance, Ending, Assets                                                                                                                                                                 | 2,211          | 1,906          | 2,211 | 1,906 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | (1)            | 2 | (8) |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 89             | (66)           | 49 | (57) |
| Corporate and other bonds                                                                                                                                                               |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance, Beginning, Assets                                                                                                                                                              | 1,129          | 971            | 1,045 | 810 |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | 0              | 0 | 0 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 59             | (36)           | 39 | (27) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 59             | (36)           | 39 | (27) |
| Purchases                                                                                                                                                                               | 83             | 29             | 229 | 178 |
| Sales                                                                                                                                                                                   | (10)           | 0              | (10) | 0 |
| Settlements                                                                                                                                                                             | (24)           | (19)           | (77) | (27) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 11 | 11 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | 0 | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 1,237          | 945            | 1,237 | 945 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 57             | (36)           | 34 | (27) |
| States, municipalities and political subdivisions                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance, Beginning, Assets                                                                                                                                                              | 43             | 43             | 44 | 43 |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | 0              | 0 | 0 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 2              | (2)            | 1 | (2) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 2              | (2)            | 1 | (2) |
| Purchases                                                                                                                                                                               | 0              | 0              | 0 | 0 |
| Sales                                                                                                                                                                                   | 0              | 0              | 0 | 0 |
| Settlements                                                                                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | 0 | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 45             | 41             | 45 | 41 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 2              | (2)            | 1 | (2) |
| Asset-backed                                                                                                                                                                            |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance, Beginning, Assets                                                                                                                                                              | 887            | 883            | 901 | 788 |
| Reported in Net investment gains (losses)                                                                                                                                               | (3)            | (4)            | (10) | (5) |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 30             | (28)           | 14 | (28) |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 31             | (27)           | 19 | (18) |
| Purchases                                                                                                                                                                               | 38             | 61             | 111 | 203 |
| Sales                                                                                                                                                                                   | 0              | 0              | (14) | 0 |
| Settlements                                                                                                                                                                             | (23)           | (13)           | (65) | (39) |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 0 | 23 |
| Transfers out of Level 3                                                                                                                                                                | (18)           | (7)            | (37) | (60) |
| Balance, Ending, Assets                                                                                                                                                                 | 915            | 897            | 915 | 897 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | 0              | 0 | 0 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 30             | (28)           | 14 | (28) |
| Equity securities                                                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Balance, Beginning, Assets                                                                                                                                                              | 14             | 26             | 24 | 35 |
| Reported in Net investment gains (losses)                                                                                                                                               | 0              | 0              | 0 | 0 |
| Reported in Other comprehensive income (loss)                                                                                                                                           | 0              | 0              | 0 | 0 |
| Total realized and unrealized investment gains (losses)                                                                                                                                 | 0              | (1)            | 6 | (8) |
| Purchases                                                                                                                                                                               | 0              | 0              | 3 | 0 |
| Sales                                                                                                                                                                                   | 0              | (2)            | (19) | (4) |
| Settlements                                                                                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                                                                                                  | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                                | 0              | 0              | 0 | 0 |
| Balance, Ending, Assets                                                                                                                                                                 | 14             | 23             | 14 | 23 |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period                                                                               | 0              | (1)            | 2 | (8) |
| Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period                                                               | 0              | 0              | 0 | 0 |
| Reported in Net investment income                                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment income                                                                                                                                                       | 4              | 4              | 21 | 7 |
| Reported in Net investment income | Corporate and other bonds                                                                                                                           |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment income                                                                                                                                                       | 0              | 0              | 0 | 0 |
| Reported in Net investment income | States, municipalities and political subdivisions                                                                                                   |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment income                                                                                                                                                       | 0              | 0              | 0 | 0 |
| Reported in Net investment income | Asset-backed                                                                                                                                        |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment income                                                                                                                                                       | 4              | 5              | 15 | 15 |
| Reported in Net investment income | Equity securities                                                                                                                                   |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                       |           |           |   |   |
| Reported in Net investment income                                                                                                                                                       | $ 0            | $ (1)          | $ 6 | $ (8) |

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Net Investment Income
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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Unrealized Gain (Loss) Included in Other Comprehensive Income (Loss)
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Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).
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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).
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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.
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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
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Indicates line item in statement of income or comprehensive income that includes gain (loss) from asset measured at fair value using unobservable input (level 3).
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Indicates line item in statement in which other comprehensive income (OCI) is reported that includes gain (loss) from asset measured at fair value using unobservable input (level 3).
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