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, 2024	Sep. 30, 2023
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 8,293	$ 5,806
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(35)	(82)
Fair Value	8,262	5,725
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,511	1,109
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(1)	(2)
Fair Value	1,510	1,108
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	6,782	4,697
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(34)	(80)
Fair Value	$ 6,752	$ 4,617