Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Sep. 30, 2023
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 7,563	$ 5,806
Gross Unrealized Gains	21	1
Gross Unrealized Losses	(44)	(82)
Fair Value	7,540	5,725
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	1,607	1,109
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(1)	(2)
Fair Value	1,608	1,108
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,956	4,697
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(43)	(80)
Fair Value	$ 5,932	$ 4,617