| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - Debt Securities - USD ($) $ in Millions** | Jun. 30, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | $ 10,832      | $ 10,643      |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 502           | 174           |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 14            | 128           |
| Fair Value                                                                                                                                                                | 11,320        | 10,689        |
| Corporate                                                                                                                                                                 |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 5,815         | 5,712         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 269           | 85            |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 13            | 87            |
| Fair Value                                                                                                                                                                | 6,071         | 5,710         |
| States, municipalities and political subdivisions                                                                                                                         |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 4,331         | 4,251         |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 218           | 84            |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 0             | 31            |
| Fair Value                                                                                                                                                                | 4,549         | 4,304         |
| Commercial mortgage-backed                                                                                                                                                |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 289           | 287           |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 12            | 3             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 0             | 2             |
| Fair Value                                                                                                                                                                | 301           | 288           |
| Government-sponsored enterprises                                                                                                                                          |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 283           | 316           |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 0             | 1             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 1             | 7             |
| Fair Value                                                                                                                                                                | 282           | 310           |
| United States government                                                                                                                                                  |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 97            | 67            |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 3             | 1             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 0             | 1             |
| Fair Value                                                                                                                                                                | 100           | 67            |
| Foreign government                                                                                                                                                        |          |          |
| **Debt Securities, Available-for-sale [Line Items]**                                                                                                                      |          |          |
| Cost or Amortized Cost                                                                                                                                                    | 17            | 10            |
| Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax                                                                                              | 0             | 0             |
| Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax                                                                                              | 0             | 0             |
| Fair Value                                                                                                                                                                | $ 17          | $ 10          |

| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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