Fair Value Measurements - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjustment to additional paid in capital remeasurement of conversion feature	$ 60,760
Long-term debt	200,021
Derivative liability	0
Convertible Debt | 5.00% / 7.00% Convertible Notes due August 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of debt	159,042
Long-term debt	$ 166,623