Long-Term Debt - Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Amortization of debt discount and deferred financing costs	$ 17,595	$ 12,303	$ 12,375
Term Loan Facility
Debt Instrument [Line Items]
Contractual interest expense	0	4,300	400
Amortization of debt discount and deferred financing costs	0	400	0
Total interest expense	0	4,700	400
2019 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,100	4,700	4,900
Amortization of debt discount and deferred financing costs	10,000	10,800	10,600
Total interest expense	14,100	15,500	15,500
Senior Notes | 2024 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	4,100	1,000	0
Amortization of debt discount and deferred financing costs	5,100	1,100	0
Total interest expense	9,200	2,100	0
Senior Notes | 2025 Convertible Notes
Debt Instrument [Line Items]
Contractual interest expense	1,600	0	0
Amortization of debt discount and deferred financing costs	2,300	0	0
Total interest expense	$ 3,900	$ 0	$ 0