| **Fair Value Measurements - Change in Level 3 Assets (Detail) (USD $)In Millions, unless otherwise specified**    | 3 Months Ended | 6 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | $ 15           | $ 6            | $ 4 | $ 5 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 0              | 0              | 0 | 0 |
| Purchases                                                                                                         | 0              | 0              | 0 | 1 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 1              | 0              | 12 | 0 |
| Transfers out of Level 3                                                                                          | (5)            | 0              | (5) | 0 |
| Ending balance                                                                                                    | 11             | 6              | 11 | 6 |
| Corporate Fixed Maturities                                                                                        |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 8              | 3              | 2 | 3 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 0              | 0              | 0 | 0 |
| Purchases                                                                                                         | 0              | 0              | 0 | 0 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 1              | 0              | 7 | 0 |
| Transfers out of Level 3                                                                                          | 0              | 0              | 0 | 0 |
| Ending balance                                                                                                    | 9              | 3              | 9 | 3 |
| Commercial Mortgage Backed Securities                                                                             |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 5              | 0              | 0 | 0 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 0              | 0              | 0 | 0 |
| Purchases                                                                                                         | 0              | 0              | 0 | 0 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 0              | 0              | 5 | 0 |
| Transfers out of Level 3                                                                                          | (5)            | 0              | (5) | 0 |
| Ending balance                                                                                                    | 0              | 0              | 0 | 0 |
| States, Municipalities And Political Subdivisions Fixed Maturities                                                |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 0              | 1              | 0 | 1 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 0              | 0              | 0 | 0 |
| Purchases                                                                                                         | 0              | 0              | 0 | 0 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                          | 0              | 0              | 0 | 0 |
| Ending balance                                                                                                    | 0              | 1              | 0 | 1 |
| Preferred Equities                                                                                                |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 2              | 2              | 2 | 1 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 0              | 0              | 0 | 0 |
| Purchases                                                                                                         | 0              | 0              | 0 | 1 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 0              | 0              | 0 | 0 |
| Transfers out of Level 3                                                                                          | 0              | 0              | 0 | 0 |
| Ending balance                                                                                                    | $ 2            | $ 2            | $ 2 | $ 2 |

| X                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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