Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 284,626	$ 242,394
Income tax expense related to uncertain tax positions	0	$ 3,500	$ 0
Unrecognized tax benefits, interest accrued	600
Unrecognized tax benefits, penalties accrued	900
Unrecognized tax benefits that would impact effective tax rate	3,500
Change in unrecognized tax benefits expected	3,500
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	134,300
Operating loss carryforwards subject to expiration	34,000
Operating loss carryforwards not subject to expiration	100,300
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	491,500
Operating loss carryforwards not subject to expiration	$ 2,900