Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,832	$ 6,262
Gross Unrealized Gains	3	7
Gross Unrealized Losses		(2)
Fair Value	3,835	6,267
Less: current portion	(3,604)	(4,110)
Long-term debt securities	231	2,157
U.S. government-sponsored debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,581	5,590
Gross Unrealized Gains	1	4
Gross Unrealized Losses		(2)
Fair Value	2,582	5,592
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,251	672
Gross Unrealized Gains	2	3
Fair Value	$ 1,253	$ 675

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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Amount 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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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