| **Fair Value Measurements - Change in Level 3 Assets (Detail) - USD ($) $ in Millions**                           | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | $ 21           | $ 11           | $ 20 | $ 4 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 1              | 0              | 1 | 0 |
| Purchases                                                                                                         | 22             | 0              | 22 | 0 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 0              | 19             | 1 | 31 |
| Transfers out of Level 3                                                                                          | (1)            | 0              | (1) | (5) |
| Ending balance                                                                                                    | 43             | 30             | 43 | 30 |
| Corporate Fixed Maturities                                                                                        |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 18             | 9              | 18 | 2 |
| **Total gains or losses (realized/unrealized):**                                                                  |           |           |   |   |
| Included in net income                                                                                            | 0              | 0              | 0 | 0 |
| Included in other comprehensive income                                                                            | 1              | 0              | 1 | 0 |
| Purchases                                                                                                         | 22             | 0              | 22 | 0 |
| Sales                                                                                                             | 0              | 0              | 0 | 0 |
| Transfers into Level 3                                                                                            | 0              | 19             | 0 | 26 |
| Transfers out of Level 3                                                                                          | 0              | 0              | 0 | 0 |
| Ending balance                                                                                                    | 41             | 28             | 41 | 28 |
| Commercial mortgage-backed                                                                                        |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 0              | 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              | 0 | 5 |
| Transfers out of Level 3                                                                                          | 0              | 0              | 0 | (5) |
| 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                                                                                                 | 1              | 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              | 1 | 0 |
| Transfers out of Level 3                                                                                          | (1)            | 0              | (1) | 0 |
| Ending balance                                                                                                    | 0              | 0              | 0 | 0 |
| Preferred Equities                                                                                                |           |           |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]** |           |           |   |   |
| Beginning balance                                                                                                 | 2              | 2              | 2 | 2 |
| **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                                                                                                    | $ 2            | $ 2            | $ 2 | $ 2 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Assets, Total Gains (Losses) Realized and Unrealized [Abstract]
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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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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