Derivative Financial Instruments - Fair Value Derivatives (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Bank loan swap agreements	$ 912	$ 52
Derivatives designated as hedging instruments | Bank loan swap agreements
Derivative [Line Items]
Bank loan swap agreements	912	52
Derivative liability	2,683	5,905
Derivatives designated as hedging instruments | Other long-term assets | Bank loan swap agreements
Derivative [Line Items]
Bank loan swap agreements	912	52
Derivatives designated as hedging instruments | Accrued other current liabilities | Bank loan swap agreements
Derivative [Line Items]
Derivative liability	155	845
Derivatives designated as hedging instruments | Derivative liabilities | Bank loan swap agreements
Derivative [Line Items]
Derivative liability	$ 2,528	$ 5,060