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Asset Retirement Obligation - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2008
Asset Retirement Obligation Disclosure [Abstract] |  |  |
Fair value of asset retirement obligations related to the legal requirements |  |  | $ 11.5 us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
Gain related to the progress of closure activities | $ 0.2 glt_GainOnDischargedAssetRetirementObligations | $ 1.3 glt_GainOnDischargedAssetRetirementObligations |

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- Definition Gain On Discharged Asset Retirement Obligations + References No definition available. + Details
Name: | glt_GainOnDischargedAssetRetirementObligations
Namespace Prefix: | glt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Definition Carrying amount of an asset that is legally restricted for purposes of settling an asset retirement obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
Name: | us-gaap_AssetRetirementObligationLegallyRestrictedAssetsFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant