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

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.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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