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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, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,140 |
Gross Unrealized Gains | 141 | $ 149
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 8
Available-for-sale Securities, Current | 2,265 | 2,949
Estimated Fair Value | 2,265 |
Available-for-sale Securities, Amortized Cost Basis | 2,140 | 2,807
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 16 | 8
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 | 0
Estimated Fair Value | 414 | 492
Available-for-sale Securities, Amortized Cost Basis | 415 | 492
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,605 | 2,175
Gross Unrealized Gains | 141 | 149
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 16 | 8
Estimated Fair Value | 1,730 | 2,317
Available-for-sale Securities, Amortized Cost Basis | 1,605 | 2,175
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 16 | 8
Fixed Income Securities [Member] | Municipal Bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 28 | 37
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 3 | 0
Estimated Fair Value | 340 | 358
Available-for-sale Securities, Amortized Cost Basis | 315 | 321
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 3 | 0
Fixed Income Securities [Member] | Corporate Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 10 | 24
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 9 | 6
Estimated Fair Value | 613 | 1,077
Available-for-sale Securities, Amortized Cost Basis | 612 | 1,059
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 9 | 6
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 0 | 1
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 2 | 0
Estimated Fair Value | 87 | 98
Available-for-sale Securities, Amortized Cost Basis | 89 | 97
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 2 | 0
Fixed Income Securities [Member] | U.S. Government Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 1 | 2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 1 | 1
Estimated Fair Value | 45 | 106
Available-for-sale Securities, Amortized Cost Basis | 45 | 105
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 1 | 1
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 182 |
Gross Unrealized Gains | 70 | 46
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 252 | 302
Available-for-sale Securities, Amortized Cost Basis | 182 | 256
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 128 |
Gross Unrealized Gains | 0 | 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 128 | 74
Available-for-sale Securities, Amortized Cost Basis | 128 | 74
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed Income Securities [Member] | Asset-backed Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 234 |
Gross Unrealized Gains | 32 | 40
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments | 0 | 0
Estimated Fair Value | 265 | 303
Available-for-sale Securities, Amortized Cost Basis | 234 | 263
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Fixed Income Investments And Other Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 141 | 149
Estimated Fair Value | 2,145 | 2,809
Available-for-sale Securities, Amortized Cost Basis | 2,020 | 2,667
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 16 | 8
Collateral Pledged [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Estimated Fair Value | 120 | 140
Collateral Pledged [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15 | 15
Gross Unrealized Gains | 0 | 0
Estimated Fair Value | 15 | 15
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | Short-term Investments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 0 | 0
Estimated Fair Value | 105 | 125
Available-for-sale Securities, Amortized Cost Basis | 105 | 125
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Treasury and Government |  |
Debt Securities, Available-for-sale [Line Items] |  |
Gross Unrealized Gains | 0 | 0
Estimated Fair Value | 15 | 15
Available-for-sale Securities, Amortized Cost Basis | 15 | 15
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | 0 | 0
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Available-for-sale Securities, Amortized Cost Basis | 120 | 140
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 | $ 0

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