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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