Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 1,033	$ 1,033	$ 2,066	$ 2,066
Amortization of debt discount and deferred financing costs	1,572	1,457	3,114	2,886
Total interest expense	2,605	2,490	5,180	4,952
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	1,024	2,048	2,048
Amortization of debt discount and deferred financing costs	1,853	1,722	3,672	3,413
Total interest expense	$ 2,877	$ 2,746	$ 5,720	$ 5,461