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Derivative Instruments (Tables) | 9 Months Ended
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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 | )

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- Definition Tabular disclosure of the location and amount of derivative instruments and nonderivative instruments designated as hedging instruments reported before netting adjustments, and the amount of gain (loss) on derivative instruments and nonderivative instruments designated and qualified as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624181-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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