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, 2019	Sep. 30, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,262	$ 7,532
Gross Unrealized Gains	7
Gross Unrealized Losse	2	16
Fair Value	6,267	7,516
Less: current portion	(4,110)	(3,434)
Long-term debt securities	2,157	4,082
U.S. government-sponsored debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,590	5,016
Gross Unrealized Gains	4
Gross Unrealized Losse	2	8
Fair Value	5,592	5,008
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	672	2,516
Gross Unrealized Gains	3
Gross Unrealized Losse		8
Fair Value	$ 675	$ 2,508

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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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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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