Fair Value Measurements and Investments - Schedule of Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 3,161	$ 5,412
Gross Unrealized Gains	19	44
Gross Unrealized Losses	0	(5)
Fair Value	3,180	5,451
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	303	492
Gross Unrealized Gains	2	4
Gross Unrealized Losses	0	0
Fair Value	305	496
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	2,858	4,920
Gross Unrealized Gains	17	40
Gross Unrealized Losses	0	(5)
Fair Value	$ 2,875	$ 4,955