Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 457,081	$ 1,278,422
Gross Unrealized Losses	(6,870)	(14,660)
Gross Unrealized Gains		22
Fair Value	450,211	1,263,784
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	227,804	804,774
Gross Unrealized Losses	(2,573)	(6,163)
Gross Unrealized Gains		21
Fair Value	225,231	798,632
Bank certificates of deposits
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,122	8,335
Gross Unrealized Losses	(170)	(158)
Gross Unrealized Gains		1
Fair Value	7,952	8,178
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	221,155	406,270
Gross Unrealized Losses	(4,127)	(8,113)
Fair Value	$ 217,028	398,157
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		59,043
Gross Unrealized Losses		(226)
Fair Value		$ 58,817