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	Jun. 30, 2024	Sep. 30, 2023
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 6,479	$ 5,806
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(28)	(82)
Fair Value	6,455	5,725
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	791	1,109
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	(2)
Fair Value	789	1,108
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,688	4,697
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(26)	(80)
Fair Value	$ 5,666	$ 4,617