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Derivative Instruments (Tables)
9 Months Ended
Sep. 30, 2018
Derivative Instruments [Abstract]  
Summary of volume of GRE's outstanding contracts and options
Settlement Dates Volume 
  Electricity (In MWH)  Gas (in Dth) 
October 2018  96,960    
November 2018  68,880   538,650 
December 2018  86,800   554,600 
January 2019  173,360   812,530 
February 2019  157,120   694,825 
March 2019  36,960   627,875 
April 2019  3,520   36,175 
May 2019  3,520   30,450 
June 2019  3,200   24,975 
July 2019  7,040   20,100 
August 2019  7,040   18,200 
September 2019  3,200   17,750 
October 2019  3,680   16,900 
November 2019  3,200   18,575 
December 2019  3,360   18,625 
Calendar 2020     112,375 
Calendar 2021     19,850 
Schedule of fair value of outstanding derivative instruments recorded as assets and liability
Asset Derivatives Balance Sheet Location September 30,
2018
  December 31, 
2017
 
    (in thousands) 
Derivatives not designated or not qualifying as hedging instruments:          
Energy contracts and options Other current assets $2,485  $4,358 
           
Liability Derivatives        
      
Derivatives not designated or not qualifying as hedging instruments:          
Energy contracts and options Other current liabilities $701  $1,228 
Schedule of derivative instruments on the consolidated statements of income
  Amount of Gain (Loss) Recognized on Derivatives 
Derivatives not designated or not qualifying as Location of Gain (Loss) Recognized 

Three Months Ended

September 30,

  

Nine Months Ended

September 30,

 
hedging instruments on Derivatives 2018  2017  2018  2017 
    (in thousands) 
Energy contracts and options  Cost of revenues $1,462  $(763) $1,911  $(3,482)