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Energy and Related Sales, Net - Summary of Energy and Related Sales Net (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Energy And Related Sales Net [Abstract]
Energy sales	$ 11,886us-gaap_SalesRevenueEnergyServices	$ 8,189us-gaap_SalesRevenueEnergyServices	$ 5,284us-gaap_SalesRevenueEnergyServices
Costs to produce	(6,204)us-gaap_CostOfServicesEnergyServices	(6,784)us-gaap_CostOfServicesEnergyServices	(4,187)us-gaap_CostOfServicesEnergyServices
Net	5,682glt_SalesRevenueEnergyServicesNet	1,405glt_SalesRevenueEnergyServicesNet	1,097glt_SalesRevenueEnergyServicesNet
Renewable energy credits	2,245us-gaap_OtherAlternativeEnergySalesRevenue	1,748us-gaap_OtherAlternativeEnergySalesRevenue	5,903us-gaap_OtherAlternativeEnergySalesRevenue
Total	$ 7,927glt_EnergyAndRelatedSalesNet	$ 3,153glt_EnergyAndRelatedSalesNet	$ 7,000glt_EnergyAndRelatedSalesNet

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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X

- Details

Name:	glt_EnergyAndRelatedSalesNetAbstract
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X

- Definition

Sales revenue energy services net.

+ References

No definition available.

+ Details

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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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X

- Definition

Revenue earned during the period from alternative energy sources (for example, ethanol, biomass, solar, geothermal, and wind power).

+ 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



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 23

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62136-109447



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62246-109447

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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



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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