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Long-Term Debt - Equipment Loans (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Long-term Debt $ 488,887 $ 469,736
Cost of funds rate 10.00%  
Equipment Loans [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount $ 31,100  
Long-term Debt $ 27,700  
Stated percentage 0.00%  
Long-term Debt, Maturities, Weighted Average of Monthly Repayments $ 1,000  
Equipment Loans [Member] | Minimum    
Debt Instrument [Line Items]    
Debt instrument, term (in months) 6 months  
Stated percentage 0.00%  
Equipment Loans [Member] | Maximum    
Debt Instrument [Line Items]    
Debt instrument, term (in months) 36 months  
Stated percentage 800.00%