Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Sep. 30, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 569,025	$ 1,278,422
Gross Unrealized Losses	(9,112)	(14,660)
Gross Unrealized Gains	1	22
Fair Value	559,914	1,263,784
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	311,618	804,774
Gross Unrealized Losses	(3,731)	(6,163)
Gross Unrealized Gains	1	21
Fair Value	307,888	798,632
Bank certificates of deposits
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,078	8,335
Gross Unrealized Losses	(194)	(158)
Gross Unrealized Gains		1
Fair Value	8,884	8,178
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	248,309	406,270
Gross Unrealized Losses	(5,186)	(8,113)
Fair Value	243,123	398,157
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20	59,043
Gross Unrealized Losses	(1)	(226)
Fair Value	$ 19	$ 58,817