Fair Value Measurements - Schedule of Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 499,046	$ 578,276
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities	499,046	578,276
Cash equivalents	30,286	66,966
Total financial assets	529,332	645,242
Fair Value, Measurements, Recurring | Money market instruments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	27,290	66,966
Fair Value, Measurements, Recurring | Commercial notes
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	2,996
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	135,718	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	136,944	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	226,384	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	27,290	66,966
Total financial assets	27,290	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	27,290	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	499,046	578,276
Cash equivalents	2,996	0
Total financial assets	502,042	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	2,996
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	135,718	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	136,944	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	226,384	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