Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - Level 2 - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Prepaid expenses and other current assets | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 0	$ 2,552	$ 0
Other assets | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0	16,042
Current maturities of long-term debt, net | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	0	(2,552)	0
Long-term debt, net | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	$ 0	$ 0	$ (16,042)