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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, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,807 | $ 3,187
Gross Unrealized Gains | 149 | 132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 8 | 5
Available-for-sale Securities, Current | 2,949 |
Estimated Fair Value | 2,949 | 3,314
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 2,807 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 8 |
Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 492 | 653
Gross Unrealized Gains | 0 |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 492 |
Available-for-sale Securities, Amortized Cost Basis | 492 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,175 | 2,450
Gross Unrealized Gains | 149 | 132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 8 | 5
Estimated Fair Value | 2,317 | 2,577
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 2,175 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 8 |
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 194
Gross Unrealized Gains | 37 | 22
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 358 | 215
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 321 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 1,396
Gross Unrealized Gains | 24 | 36
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 6 | 2
Estimated Fair Value | 1,077 | 1,430
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 1,059 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 6 |
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 44
Gross Unrealized Gains | 1 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 98 | 44
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 97 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 157
Gross Unrealized Gains | 2 | 2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 2
Estimated Fair Value | 106 | 156
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 105 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 1 |
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 256 | 200
Gross Unrealized Gains | 46 | 47
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 302 | 248
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 256 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 74 | 147
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 1
Estimated Fair Value | 74 | 146
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 74 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 263 | 263
Gross Unrealized Gains | 40 | 24
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 303 | 287
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 263 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 653
Gross Unrealized Gains |  | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Estimated Fair Value |  | 653
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] | Commercial 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 Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 3,103
Gross Unrealized Gains | 149 | 132
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 5
Estimated Fair Value | 2,809 | 3,230
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities |  | 0
Available-for-sale Securities, Amortized Cost Basis | 2,667 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 8 |
Collateral Pledged [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 140 |
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15 | 0
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Estimated Fair Value | 15 | 85
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  | 0
Estimated Fair Value | 125 | 85
Available-for-sale Securities, Amortized Cost Basis | 125 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Treasury and Government |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 0 |
Estimated Fair Value | 15 |
Available-for-sale Securities, Amortized Cost Basis | 15 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 |
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 85
Available-for-sale Securities, Amortized Cost Basis | 140 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 |
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | $ 85

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- Definition Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 9A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284427-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580 + Details
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