Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) $ in Thousands	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2014
Prepaid expenses and other current assets | Recurring | Level 2 | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 21	$ 2,552	$ 0
Other assets | Recurring | Level 2 | 2019 Convertible Notes Hedges — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0	12,863
Current maturities of long-term-debt, net | Recurring | Level 2 | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	(21)	(2,552)	0
Long-term debt, net | Recurring | Level 2 | 2019 Convertible Notes Embedded Derivative — Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	$ 0	$ 0	$ (12,863)
Price Volatility | Recurring | Convertible Hedges and Embedded Derivative Financial Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative measurement input	0.36
2019 Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Stated interest rate				2.125%