Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 1,033	$ 1,033	$ 3,100	$ 3,100
Amortization of debt discount and deferred financing costs	1,602	1,484	4,716	4,370
Total interest expense	2,635	2,517	7,816	7,470
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	1,024	3,073	3,072
Amortization of debt discount and deferred financing costs	1,886	1,754	5,558	5,168
Total interest expense	$ 2,910	$ 2,778	$ 8,631	$ 8,240