Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 0	$ 1,069	$ 0	$ 2,138
Amortization of debt discount and deferred financing costs	0	2,610	0	5,253
Total interest expense	0	3,679	0	7,391
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	1,457	1,350	2,886	2,675
Total interest expense	2,490	2,383	4,952	4,741
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	1,722	1,602	3,413	3,176
Total interest expense	$ 2,746	$ 2,626	$ 5,461	$ 5,224