Fair value measurement - Summary of Company's Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Assets
Total	$ 266,269	$ 286,470
Non-current assets - investment in equity securities	3,711	3,711
Total	283,265	299,308
Liabilities
Total financial liabilities	0	0
U.S. government treasury bills
Assets
Short term investments, fair value	38,904	35,054
Corporate bonds
Assets
Short term investments, fair value		14,889
Money market funds
Assets
Total	234,679	245,654
Corporate bonds
Assets
Total	5,971
Level 1
Assets
Non-current assets - investment in equity securities	0	0
Total	273,583	280,708
Liabilities
Total financial liabilities	0	0
Level 1 | U.S. government treasury bills
Assets
Short term investments, fair value	38,904	35,054
Level 1 | Corporate bonds
Assets
Short term investments, fair value		0
Level 1 | Money market funds
Assets
Total	234,679	245,654
Level 1 | Corporate bonds
Assets
Total	0
Level 2
Assets
Non-current assets - investment in equity securities	0	0
Total	5,971	14,889
Liabilities
Total financial liabilities	0	0
Level 2 | U.S. government treasury bills
Assets
Short term investments, fair value	0	0
Level 2 | Corporate bonds
Assets
Short term investments, fair value		14,889
Level 2 | Money market funds
Assets
Total	0	0
Level 2 | Corporate bonds
Assets
Total	5,971
Level 3
Assets
Non-current assets - investment in equity securities	3,711	3,711
Total	3,711	3,711
Liabilities
Total financial liabilities	0	0
Level 3 | U.S. government treasury bills
Assets
Short term investments, fair value	0	0
Level 3 | Corporate bonds
Assets
Short term investments, fair value		0
Level 3 | Money market funds
Assets
Total	0	$ 0
Level 3 | Corporate bonds
Assets
Total	$ 0