Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 613,712	$ 457,081
Gross Unrealized Losses	(2,485)	(6,870)
Gross Unrealized Gains	11
Fair Value	611,238	450,211
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	433,188	227,804
Gross Unrealized Losses	(902)	(2,573)
Gross Unrealized Gains	8
Fair Value	432,294	225,231
Bank certificates of deposits
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,870	8,122
Gross Unrealized Losses	(68)	(170)
Fair Value	7,802	7,952
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	171,648	221,155
Gross Unrealized Losses	(1,515)	(4,127)
Gross Unrealized Gains	3
Fair Value	170,136	$ 217,028
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,006
Fair Value	$ 1,006