Leases - Components of Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Lease Assets and Liabilities
Right-of-use assets	$ 45,297	$ 58,291
Lease liabilities	10,563	2,776
Lease liabilities, net of current portion	104,608	78,800
Lease Cost
Total	13,259	9,763	$ 5,962
Research and development
Lease Cost
Operating lease cost	10,350	7,278	3,649
Variable lease cost	1,179	728	814
General and administrative expense
Lease Cost
Operating lease cost	1,730	1,757	1,498
Variable lease cost	$ 0	$ 0	$ 1