Fair Value Measurements - Schedule of Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 911,700	$ 578,276
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	911,700	578,276
Cash equivalents	166,443	66,966
Total financial assets	1,078,143	645,242
Fair Value, Measurements, Recurring | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	137,535	66,966
Fair Value, Measurements, Recurring | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	28,908
Fair Value, Measurements, Recurring | U.S. government and agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	277,170	160,723
Fair Value, Measurements, Recurring | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	113,387	179,714
Fair Value, Measurements, Recurring | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	521,143	237,839
Fair Value, Measurements, Recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Cash equivalents	137,535	66,966
Total financial assets	137,535	66,966
Fair Value, Measurements, Recurring | Level 1 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	137,535	66,966
Fair Value, Measurements, Recurring | Level 1 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring | Level 1 | U.S. government and agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	911,700	578,276
Cash equivalents	28,908	0
Total financial assets	940,608	578,276
Fair Value, Measurements, Recurring | Level 2 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 2 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	28,908
Fair Value, Measurements, Recurring | Level 2 | U.S. government and agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	277,170	160,723
Fair Value, Measurements, Recurring | Level 2 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	113,387	179,714
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	521,143	237,839
Fair Value, Measurements, Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Cash equivalents	0	0
Total financial assets	0	0
Fair Value, Measurements, Recurring | Level 3 | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Fair Value, Measurements, Recurring | Level 3 | U.S. government and agency securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0	$ 0