Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 789,025	$ 1,278,422
Gross Unrealized Losses	(9,230)	(14,660)
Gross Unrealized Gains	32	22
Fair Value	779,827	1,263,784
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	435,258	804,774
Gross Unrealized Losses	(3,672)	(6,163)
Gross Unrealized Gains	31	21
Fair Value	431,617	798,632
Bank certificates of deposits
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,355	8,335
Gross Unrealized Losses	(187)	(158)
Gross Unrealized Gains	1	1
Fair Value	10,169	8,178
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	341,868	406,270
Gross Unrealized Losses	(5,370)	(8,113)
Fair Value	336,498	398,157
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,544	59,043
Gross Unrealized Losses	(1)	(226)
Fair Value	$ 1,543	$ 58,817

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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