Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Amortization of debt discount and deferred financing costs	$ 13,200	$ 19,759	$ 17,595
Senior Notes | 2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	0	3,074	4,144
Amortization of debt discount and deferred financing costs	0	7,556	9,952
Total interest expense	0	10,630	14,096
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	5,883	5,452	5,057
Total interest expense	10,016	9,585	9,190
Senior Notes | 2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,097	4,097	1,559
Amortization of debt discount and deferred financing costs	6,954	6,468	2,383
Total interest expense	$ 11,051	$ 10,565	$ 3,942