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Derivative Instruments and Fair Value Measurements - Use of derivative instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Derivative, Fair Value, Net [Abstract]    
As reported fair value contracts $ 166 $ 112
As reported fair value contracts 145 64
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Current Assets [Member]    
Derivative, Fair Value, Net [Abstract]    
Asset derivatives: 166 378
Liability derivatives: 0 0
Less counterparty offsets 0 (266)
As reported fair value contracts 166 112
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]    
Derivative, Fair Value, Net [Abstract]    
Asset derivatives: 0 0
Liability derivatives: 0 0
Less counterparty offsets 0 0
As reported fair value contracts 0 0
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Current Liabilities [Member]    
Derivative, Fair Value, Net [Abstract]    
Asset derivatives: 0 0
Liability derivatives: 145 330
Less counterparty offsets 0 (266)
As reported fair value contracts 145 64
Commodity Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Liabilities [Member]    
Derivative, Fair Value, Net [Abstract]    
Asset derivatives: 0 0
Liability derivatives: 0 0
Less counterparty offsets 0 0
As reported fair value contracts $ 0 $ 0