Debt - Interest Expense (Details) - Senior Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 0	$ 896	$ 0	$ 3,034
Amortization of debt discount and deferred financing costs	0	2,314	0	7,567
Total interest expense	0	3,210	0	10,601
2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,033	1,033	3,100	3,099
Amortization of debt discount and deferred financing costs	1,484	1,376	4,370	4,051
Total interest expense	2,517	2,409	7,470	7,150
2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,024	1,024	3,072	3,072
Amortization of debt discount and deferred financing costs	1,754	1,630	5,168	4,806
Total interest expense	$ 2,778	$ 2,654	$ 8,240	$ 7,878