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	Sep. 30, 2025	Sep. 30, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,405	$ 5,412
Gross Unrealized Gains	16	44
Gross Unrealized Losses	0	(5)
Fair Value	2,421	5,451
U.S. government-sponsored debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	304	492
Gross Unrealized Gains	1	4
Gross Unrealized Losses	0	0
Fair Value	305	496
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,101	4,920
Gross Unrealized Gains	15	40
Gross Unrealized Losses	0	(5)
Fair Value	$ 2,116	$ 4,955