Fair value measurement - Cash And Cash Equivalents And Available For Sale Securities At Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 315,196	$ 378,687
Amortized cost	387,547	506,562
Gross unrealized gains	134	0
Gross unrealized losses	(23)	(1,594)
Cash and cash equivalents, fair value	315,196	378,687
Fair value	387,658	504,968
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments, Amortized cost	33,359
Gross unrealized gains	0
Gross unrealized losses	(23)
Short term investments, fair value	33,336
U.S. government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments, Amortized cost	38,992	112,878
Gross unrealized gains	134	0
Gross unrealized losses	0	(1,594)
Short term investments, fair value	$ 39,126	111,284
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments, Amortized cost		14,997
Gross unrealized gains		0
Gross unrealized losses		0
Short term investments, fair value		$ 14,997