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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash collateral (deficit) $ 290 $ (630)
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets   9
Net assets (liabilities) (1,087) (73)
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets   0
Net assets (liabilities) 0 0
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets   9
Net assets (liabilities) (1,087) (73)
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets   0
Net assets (liabilities) 0 0
Fair Value, Measurements, Recurring | OTC commodity swaps    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset   9
Derivative Liability (1,087) (82)
Fair Value, Measurements, Recurring | OTC commodity swaps | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset   0
Derivative Liability 0 0
Fair Value, Measurements, Recurring | OTC commodity swaps | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset   9
Derivative Liability (1,087) (82)
Fair Value, Measurements, Recurring | OTC commodity swaps | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset   0
Derivative Liability $ 0 $ 0