Investments - Summary of Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,013	$ 2,872
Gross Unrealized Gains	4	11
Gross Unrealized Losses	(2)	0
Estimated Fair Value	3,015	2,883
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	553	406
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	553	406
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,261	2,298
Gross Unrealized Gains	4	10
Gross Unrealized Losses	(1)	0
Estimated Fair Value	2,264	2,308
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	40	23
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	40	23
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	97	145
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	0
Estimated Fair Value	97	$ 146
Mortgage and asset backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	62
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Estimated Fair Value	$ 61

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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Amortized cost of 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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