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Plant, Equipment and Timberlands - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Property, Plant and Equipment [Abstract] |  |
Accrued capital expenditures | $ 16.3 glt_AccruedCapitalExpenditures | $ 11.9 glt_AccruedCapitalExpenditures

X
- Definition Accrued Capital Expenditures + References No definition available. + Details
Name: | glt_AccruedCapitalExpenditures
Namespace Prefix: | glt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration