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Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 5,348	$ 5,395
Gross Unrealized Gains	7	0
Gross Unrealized Losses	(90)	(134)
Fair Value	5,265	5,261
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,016	458
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1)	(1)
Fair Value	1,016	457
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	4,332	4,937
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(89)	(133)
Fair Value	$ 4,249	$ 4,804

X

- Definition

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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