Fair Value Measurements and Investments - Continuous Unrealized Losses for Less than 12 Months and More than 12 Months (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	$ 4,071	$ 1,772
Less Than 12 Months, Gross Unrealized Losses	(7)	(14)
12 Months or Greater, Fair Value	1,870	2,178
12 Months or Greater, Gross Unrealized Losses	(28)	(68)
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	1,282	412
Less Than 12 Months, Gross Unrealized Losses	(1)	(2)
12 Months or Greater, Fair Value	0	50
12 Months or Greater, Gross Unrealized Losses	0	0
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	2,789	1,360
Less Than 12 Months, Gross Unrealized Losses	(6)	(12)
12 Months or Greater, Fair Value	1,870	2,128
12 Months or Greater, Gross Unrealized Losses	$ (28)	$ (68)