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Energy and Related Sales, Net - Summary of Energy and Related Sales Net (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Energy And Related Sales Net [Abstract]
Energy sales	$ 5,315	$ 11,886	$ 8,189
Costs to produce	(4,428)	(6,204)	(6,784)
Net	887	5,682	1,405
Renewable energy credits	4,777	2,245	1,748
Total	$ 5,664	$ 7,927	$ 3,153

X

- Definition

Revenue from sales of excess internally generated power, net of costs to produce, and revenue from sales of renewable energy credits.

+ References

No definition available.

+ Details

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- Definition

Energy and Related Sales, Net [Abstract]

+ References

No definition available.

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- Definition

Renewable Energy Credits Sales Revenue

+ References

No definition available.

+ Details

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X

- Definition

Sales revenue energy services net.

+ References

No definition available.

+ Details

Name:	glt_SalesRevenueEnergyServicesNet
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- Definition

Cost incurred in providing energy services during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(d))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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- Definition

Revenue derived from energy-related services such as supplying various forms of energy from the operation of on-site facilities or cogeneration plants and procurement services provided to energy purchasers.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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