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Derivative Instruments (Details)	3 Months Ended
	Mar. 31, 2017
Electricity [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	April 2017
Volume	16,000 MWh
Electricity 1 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	June 2017
Volume	153,120 MWh
Electricity 2 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	July 2017
Volume	334,000 MWh
Electricity 3 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	August 2017
Volume	420,900 MWh
Electricity 4 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	September 2017
Volume	75,200 MWh
Electricity 5 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	October 2017
Volume	241,120 MWh
Electricity 6 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	November 2017
Volume	213,360 MWh
Electricity 7 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	December 2017
Volume	283,200 MWh
Electricity 8 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	January 2018
Volume	362,560 MWh
Electricity 9 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	February 2018
Volume	329,600 MWh
Electricity Ten [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	March 2018
Volume	179,520 MWh
Electricity 11 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	April 2018
Volume	67,200 MWh
Electricity 12 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	May 2018
Volume	70,400 MWh
Electricity 13 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	June 2018
Volume	67,200 MWh
Electricity 14 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	July 2018
Volume	67,200 MWh
Electricity 15 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	August 2018
Volume	73,600 MWh
Electricity 16 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	September 2018
Volume	60,800 MWh
Electricity 17 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	October 2018
Volume	147,200 MWh
Electricity 18 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	November 2018
Volume	134,400 MWh
Electricity 19 [Member]
Summarized volume of GRE's outstanding
Commodity	Electricity
Settlement Dates	December 2018
Volume	128,000 MWh
Natural Gas [Member]
Summarized volume of GRE's outstanding
Commodity	Natural gas
Settlement Dates	October 2017
Volume	200,000 Dth
Natural Gas 1 [Member]
Summarized volume of GRE's outstanding
Commodity	Natural gas
Settlement Dates	November 2017
Volume	1,100,000 Dth
Natural Gas 2 [Member]
Summarized volume of GRE's outstanding
Commodity	Natural gas
Settlement Dates	December 2017
Volume	700,000 Dth
Natural Gas 3 [Member]
Summarized volume of GRE's outstanding
Commodity	Natural gas
Settlement Dates	February 2018
Volume	888,400 Dth
Natural Gas 4 [Member]
Summarized volume of GRE's outstanding
Commodity	Natural gas
Settlement Dates	March 2018
Volume	600,000 Dth

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959

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