Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 0	$ 1,076
Amortization of debt discount and deferred financing costs	0	2,643
Total interest expense	0	3,719
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,033	1,033
Amortization of debt discount and deferred financing costs	1,429	1,325
Total interest expense	2,462	2,358
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	1,024
Amortization of debt discount and deferred financing costs	1,691	1,573
Total interest expense	$ 2,715	$ 2,597