| **Fair Value - Reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | $ 443          | $ 491          | $ 407 | $ 473 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 1              | 8              | 8 | 12 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 2              | 4              | 6 | 7 |
| Purchases                                                                                                                                                                       | 13             | 139            | 57 | 201 |
| Sales                                                                                                                                                                           | (46)           | (45)           | (63) | (61) |
| Settlements                                                                                                                                                                     | (18)           | (32)           | (43) | (46) |
| Transfers into Level 3                                                                                                                                                          | 24             | 3              | 52 | 5 |
| Transfers out of Level 3                                                                                                                                                        | (80)           | (48)           | (85) | (71) |
| Balance, end of year                                                                                                                                                            | 339            | 520            | 339 | 520 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | (3)            | 0 | (3) |
| Corporate and other bonds                                                                                                                                                       |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 121            | 193            | 130 | 168 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 1              | 0 | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 0              | 3              | 1 | 7 |
| Purchases                                                                                                                                                                       | 0              | 94             | 5 | 147 |
| Sales                                                                                                                                                                           | 0              | (20)           | (1) | (36) |
| Settlements                                                                                                                                                                     | (11)           | (7)            | (25) | (10) |
| Transfers into Level 3                                                                                                                                                          | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                        | (10)           | (22)           | (10) | (34) |
| Balance, end of year                                                                                                                                                            | 100            | 242            | 100 | 242 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| States, municipalities and political subdivisions                                                                                                                               |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 1              | 2              | 1 | 2 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 0              | 0 | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 0              | 0              | 0 | 0 |
| 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, end of year                                                                                                                                                            | 1              | 2              | 1 | 2 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Residential mortgage-backed                                                                                                                                                     |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 126            | 128            | 129 | 134 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 1              | 1              | 2 | 2 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 1              | (1)            | 3 | (1) |
| Purchases                                                                                                                                                                       | 0              | 10             | 0 | 10 |
| Sales                                                                                                                                                                           | 0              | 0              | 0 | 0 |
| Settlements                                                                                                                                                                     | (5)            | (4)            | (11) | (9) |
| Transfers into Level 3                                                                                                                                                          | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                        | 0              | 0              | 0 | (2) |
| Balance, end of year                                                                                                                                                            | 123            | 134            | 123 | 134 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Commercial mortgage-backed                                                                                                                                                      |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 13             | 27             | 13 | 22 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 0              | 0 | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 0              | 0              | 0 | 0 |
| Purchases                                                                                                                                                                       | 0              | 0              | 0 | 9 |
| Sales                                                                                                                                                                           | 0              | 0              | 0 | 0 |
| Settlements                                                                                                                                                                     | 0              | (9)            | 0 | (9) |
| Transfers into Level 3                                                                                                                                                          | 0              | 3              | 0 | 3 |
| Transfers out of Level 3                                                                                                                                                        | 0              | (10)           | 0 | (14) |
| Balance, end of year                                                                                                                                                            | 13             | 11             | 13 | 11 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Other asset-backed                                                                                                                                                              |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 117            | 50             | 57 | 53 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 0              | (1) | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 0              | 2              | 0 | 2 |
| Purchases                                                                                                                                                                       | 13             | 35             | 51 | 35 |
| Sales                                                                                                                                                                           | 0              | (25)           | 0 | (25) |
| Settlements                                                                                                                                                                     | (2)            | (1)            | (2) | (1) |
| Transfers into Level 3                                                                                                                                                          | 24             | 0              | 52 | 2 |
| Transfers out of Level 3                                                                                                                                                        | (70)           | (16)           | (75) | (21) |
| Balance, end of year                                                                                                                                                            | 82             | 45             | 82 | 45 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Total asset-backed                                                                                                                                                              |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 256            | 205            | 199 | 209 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 1              | 1              | 1 | 2 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 1              | 1              | 3 | 1 |
| Purchases                                                                                                                                                                       | 13             | 45             | 51 | 54 |
| Sales                                                                                                                                                                           | 0              | (25)           | 0 | (25) |
| Settlements                                                                                                                                                                     | (7)            | (14)           | (13) | (19) |
| Transfers into Level 3                                                                                                                                                          | 24             | 3              | 52 | 5 |
| Transfers out of Level 3                                                                                                                                                        | (70)           | (26)           | (75) | (37) |
| Balance, end of year                                                                                                                                                            | 218            | 190            | 218 | 190 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Total fixed maturity securities                                                                                                                                                 |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 378            | 400            | 330 | 379 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 1              | 2              | 1 | 2 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 1              | 4              | 4 | 8 |
| Purchases                                                                                                                                                                       | 13             | 139            | 56 | 201 |
| Sales                                                                                                                                                                           | 0              | (45)           | (1) | (61) |
| Settlements                                                                                                                                                                     | (18)           | (21)           | (38) | (29) |
| Transfers into Level 3                                                                                                                                                          | 24             | 3              | 52 | 5 |
| Transfers out of Level 3                                                                                                                                                        | (80)           | (48)           | (85) | (71) |
| Balance, end of year                                                                                                                                                            | 319            | 434            | 319 | 434 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Equity securities                                                                                                                                                               |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 19             | 19             | 19 | 20 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 0              | 0 | 0 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 1              | 0              | 2 | (1) |
| Purchases                                                                                                                                                                       | 0              | 0              | 1 | 0 |
| Sales                                                                                                                                                                           | (1)            | 0              | (3) | 0 |
| Settlements                                                                                                                                                                     | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                                                                                          | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                        | 0              | 0              | 0 | 0 |
| Balance, end of year                                                                                                                                                            | 19             | 19             | 19 | 19 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | 0              | 0              | 0 | 0 |
| Derivative financial instruments                                                                                                                                                |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      |           |           | 0 |   |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   |           |           | 1 |   |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              |           |           | 0 |   |
| Purchases                                                                                                                                                                       |           |           | 0 |   |
| Sales                                                                                                                                                                           |           |           | (1) |   |
| Settlements                                                                                                                                                                     |           |           | 0 |   |
| Transfers into Level 3                                                                                                                                                          |           |           | 0 |   |
| Transfers out of Level 3                                                                                                                                                        |           |           | 0 |   |
| Balance, end of year                                                                                                                                                            | 0              |           | 0 |   |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     |           |           | 0 |   |
| Life settlement contracts, included in Other assets                                                                                                                             |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                                      | 46             | 72             | 58 | 74 |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                   | 0              | 6              | 6 | 10 |
| Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)                                                                              | 0              | 0              | 0 | 0 |
| Purchases                                                                                                                                                                       | 0              | 0              | 0 | 0 |
| Sales                                                                                                                                                                           | (45)           | 0              | (58) | 0 |
| Settlements                                                                                                                                                                     | 0              | (11)           | (5) | (17) |
| Transfers into Level 3                                                                                                                                                          | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                                                                                        | 0              | 0              | 0 | 0 |
| Balance, end of year                                                                                                                                                            | 1              | 67             | 1 | 67 |
| Unrealized gains (losses) on Level 3 assets and liabilities, ending balance                                                                                                     | $ 0            | $ (3)          | $ 0 | $ (3) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258
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| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for 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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in other comprehensive income (loss) for 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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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.
+ References
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| - Definition
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.
+ References
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| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
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| - Definition
Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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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