Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Convertible Notes, Due 2019
Debt Instrument [Line Items]
Contractual interest expense	$ 1,069	$ 1,069	$ 2,138	$ 2,138
Amortization of debt discount and deferred financing costs	2,610	2,455	5,253	4,943
Total interest expense	3,679	3,524	7,391	7,081
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	1,350	1,250	2,675	2,481
Total interest expense	2,383	2,283	4,741	4,547
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	0	2,048	0
Amortization of debt discount and deferred financing costs	1,602	0	3,176	0
Total interest expense	$ 2,626	$ 0	$ 5,224	$ 0