| **Accumulated Other Comprehensive Income (Loss) by Component - Changes in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               |           |           | $ 11,969 |   |
| Other comprehensive income (loss) before reclassifications                                                                                                               | $ 149          | $ 274          | 245 | $ 514 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit                                                                      | 25             | 8              | 40 | (11) |
| Other comprehensive income (loss) after tax (expense) benefit                                                                                                            | 124            | 266            | 205 | 525 |
| Balance, end of period                                                                                                                                                   | 12,028         | 11,886         | 12,028 | 11,886 |
| Reclassification from AOCI, current period, tax                                                                                                                          | (11)           | (3)            | (18) | 8 |
| Tax (expense) benefit on other comprehensive income (loss)                                                                                                               | (51)           | (159)          | (92) | (281) |
| Net unrealized gains (losses) on investments with OTTI losses                                                                                                            |           |           |   |   |
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               | 26             | 32             | 30 | 27 |
| Other comprehensive income (loss) before reclassifications                                                                                                               | (1)            | (1)            | (1) | 2 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit                                                                      | (1)            | 0              | 3 | (2) |
| Other comprehensive income (loss) after tax (expense) benefit                                                                                                            | 0              | (1)            | (4) | 4 |
| Balance, end of period                                                                                                                                                   | 26             | 31             | 26 | 31 |
| Reclassification from AOCI, current period, tax                                                                                                                          | 1              | 0              | (1) | 1 |
| Tax (expense) benefit on other comprehensive income (loss)                                                                                                               | 0              | 1              | 1 | (2) |
| Net unrealized gains (losses) on other investments                                                                                                                       |           |           |   |   |
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               | 709            | 624            | 642 | 390 |
| Other comprehensive income (loss) before reclassifications                                                                                                               | 108            | 323            | 193 | 546 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit                                                                      | 31             | 13             | 49 | 2 |
| Other comprehensive income (loss) after tax (expense) benefit                                                                                                            | 77             | 310            | 144 | 544 |
| Balance, end of period                                                                                                                                                   | 786            | 934            | 786 | 934 |
| Reclassification from AOCI, current period, tax                                                                                                                          | (15)           | (6)            | (24) | 1 |
| Tax (expense) benefit on other comprehensive income (loss)                                                                                                               | (48)           | (157)          | (86) | (273) |
| Pension and postretirement benefits                                                                                                                                      |           |           |   |   |
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               | (640)          | (642)          | (647) | (648) |
| Other comprehensive income (loss) before reclassifications                                                                                                               | 0              | 0              | 0 | 0 |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit                                                                      | (5)            | (5)            | (12) | (11) |
| Other comprehensive income (loss) after tax (expense) benefit                                                                                                            | 5              | 5              | 12 | 11 |
| Balance, end of period                                                                                                                                                   | (635)          | (637)          | (635) | (637) |
| Reclassification from AOCI, current period, tax                                                                                                                          | 3              | 3              | 7 | 6 |
| Tax (expense) benefit on other comprehensive income (loss)                                                                                                               | (3)            | (3)            | (7) | (6) |
| Cumulative foreign currency translation adjustment                                                                                                                       |           |           |   |   |
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               | (187)          | (70)           | (198) | (84) |
| Other comprehensive income (loss) before reclassifications                                                                                                               | 42             | (48)           | 53 | (34) |
| Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit                                                                      | 0              | 0              | 0 | 0 |
| Other comprehensive income (loss) after tax (expense) benefit                                                                                                            | 42             | (48)           | 53 | (34) |
| Balance, end of period                                                                                                                                                   | (145)          | (118)          | (145) | (118) |
| Reclassification from AOCI, current period, tax                                                                                                                          | 0              | 0              | 0 | 0 |
| Tax (expense) benefit on other comprehensive income (loss)                                                                                                               | 0              | 0              | 0 | 0 |
| AOCI Attributable to Parent                                                                                                                                              |           |           |   |   |
| **AOCI Attributable to Parent, Net of Tax [Roll Forward]**                                                                                                               |           |           |   |   |
| Balance, beginning of year                                                                                                                                               | (92)           | (56)           | (173) | (315) |
| Balance, end of period                                                                                                                                                   | $ 32           | $ 210          | $ 32 | $ 210 |

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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 after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.
+ References
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| - Definition
Amount after tax of other comprehensive income (loss) attributable to parent entity.
+ References
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| - Definition
Amount of tax expense (benefit) allocated to other comprehensive income (loss) attributable to parent entity.
+ References
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| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.
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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).
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| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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