Fair Value Measurements and Investments - Continuous Unrealized Losses for Less than 12 Months and More than 12 Months (Details) - USD ($) $ in Millions	Dec. 31, 2022	Sep. 30, 2022
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	$ 1,945	$ 3,915
Less Than 12 Months, Gross Unrealized Losses	(39)	(134)
12 Months or Greater, Fair Value	1,966
12 Months or Greater, Gross Unrealized Losses	(83)
U.S. government-sponsored debt securities
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	347	408
Less Than 12 Months, Gross Unrealized Losses	(1)	(1)
12 Months or Greater, Fair Value	0
12 Months or Greater, Gross Unrealized Losses	0
U.S. Treasury securities
Debt Securities, Available-for-Sale [Line Items]
Less Than 12 Months, Fair Value	1,598	3,507
Less Than 12 Months, Gross Unrealized Losses	(38)	$ (133)
12 Months or Greater, Fair Value	1,966
12 Months or Greater, Gross Unrealized Losses	$ (83)