Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 1,033	$ 1,033
Amortization of debt discount and deferred financing costs	1,542	1,429
Total interest expense	2,575	2,462
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	1,024
Amortization of debt discount and deferred financing costs	1,819	1,691
Total interest expense	$ 2,843	$ 2,715