Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Convertible Notes, Due 2019
Debt Instrument [Line Items]
Contractual interest expense	$ 896	$ 1,069	$ 3,034	$ 3,207
Amortization of debt discount and deferred financing costs	2,314	2,527	7,567	7,470
Total interest expense	3,210	3,596	10,601	10,677
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,033	1,033	3,099	3,099
Amortization of debt discount and deferred financing costs	1,376	1,275	4,051	3,756
Total interest expense	2,409	2,308	7,150	6,855
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	535	3,072	535
Amortization of debt discount and deferred financing costs	1,630	836	4,806	836
Total interest expense	$ 2,654	$ 1,371	$ 7,878	$ 1,371