Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Amortization of debt discount and deferred financing costs	$ 13,797	$ 13,200	$ 19,759
Senior Notes | 2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	0	0	3,074
Amortization of debt discount and deferred financing costs	0	0	7,556
Total interest expense	0	0	10,630
Senior Notes | 2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,133	4,133	4,133
Amortization of debt discount and deferred financing costs	6,349	5,883	5,452
Total interest expense	10,482	10,016	9,585
Senior Notes | 2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,097	4,097	4,097
Amortization of debt discount and deferred financing costs	7,479	6,954	6,468
Total interest expense	$ 11,576	$ 11,051	$ 10,565