Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Amortization of debt discount and deferred financing costs	$ 19,759	$ 17,595	$ 12,303
Term Loan Facility
Debt Instrument [Line Items]
Contractual interest expense	0	0	4,345
Amortization of debt discount and deferred financing costs	0	0	359
Total interest expense	0	0	4,704
2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	3,074	4,144	4,741
Amortization of debt discount and deferred financing costs	7,556	9,952	10,759
Total interest expense	10,630	14,096	15,500
Senior Notes | 2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,133	4,133	987
Amortization of debt discount and deferred financing costs	5,452	5,057	1,067
Total interest expense	9,585	9,190	2,054
Senior Notes | 2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,097	1,559	0
Amortization of debt discount and deferred financing costs	6,468	2,383	0
Total interest expense	$ 10,565	$ 3,942	$ 0