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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 3,187 | $ 3,451
Gross Unrealized Gains | 132 | 133
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 | 38
Available-for-sale Securities, Current | 3,314 | 3,546
Estimated Fair Value | 3,314 |
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Below Investment Grade Securities and Non-Rated Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses | 0 | 18
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value | 29 | 660
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 653 | 430
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 653 | 430
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,450 | 3,021
Gross Unrealized Gains | 132 | 133
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 | 38
Estimated Fair Value | 2,577 | 3,116
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 194 | 883
Gross Unrealized Gains | 22 | 14
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 17
Estimated Fair Value | 215 | 880
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,396 | 1,289
Gross Unrealized Gains | 36 | 6
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 17
Estimated Fair Value | 1,430 | 1,278
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 44 | 30
Gross Unrealized Gains | 1 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 44 | 31
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 157 | 94
Gross Unrealized Gains | 2 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 1
Estimated Fair Value | 156 | 94
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 200 | 222
Gross Unrealized Gains | 47 | 38
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 248 | 259
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 49 |
Gross Unrealized Gains | 1 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 |
Estimated Fair Value | 50 |
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 |
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 147 | 133
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 2
Estimated Fair Value | 146 | 131
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 263 | 370
Gross Unrealized Gains | 24 | 73
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 287 | 442
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 | 0
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,103 |
Gross Unrealized Gains | 132 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 5 |
Estimated Fair Value | 3,230 |
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 |
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 85 | 0
Gross Unrealized Gains | 0 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 |
Estimated Fair Value | 85 | $ 0
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 |
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 85 |
Gross Unrealized Gains | 0 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 |
Estimated Fair Value | 85 |
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities | 0 |
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 85 |

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