Annual report pursuant to Section 13 and 15(d)

Fair Value Measurement - Fair Value and Carrying Value of Long-term Debt (Details)

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Fair Value Measurement - Fair Value and Carrying Value of Long-term Debt (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt value $ 160,108  
Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt value 160,108 $ 145,746
Carrying Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt value $ 160,598 $ 145,238