Fair Value Measurements - Recurring Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2014
Prepaid expenses and other current assets | Level 2 | Convertible Notes Hedges | Recurring
Financial Assets (Liabilities):
Derivative asset	$ 2,552	$ 0
Other assets, net | Level 2 | Convertible Notes Hedges | Recurring
Financial Assets (Liabilities):
Derivative asset	0	6,591
Current maturities of long-term debt, net | Level 2 | Convertible Notes Embedded Derivative | Recurring
Financial Assets (Liabilities):
Derivative liability	(2,552)	0
Long-term debt, net | Convertible Notes Embedded Derivative
Financial Assets (Liabilities):
Derivative liability			$ (46,500)
Long-term debt, net | Level 2 | Convertible Notes Embedded Derivative | Recurring
Financial Assets (Liabilities):
Derivative liability	$ 0	$ (6,591)