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Other Long-Term Assets - Summary of Other Long-Term Assets (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Other Assets Noncurrent [Abstract] |  |
Pension | $ 115,482 | $ 96,699
Other | 22,996 | 25,050
Total | $ 138,478 | $ 121,749

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- Definition Prepaid pension cost. + References No definition available. + Details
Name: | glt_PrepaidPensionCost
Namespace Prefix: | glt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of other miscellaneous assets expected to be realized or consumed after one year or normal operating cycle, if longer. + References No definition available. + Details
Name: | us-gaap_OtherAssetsMiscellaneousNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of noncurrent assets classified as other. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_OtherAssetsNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_OtherAssetsNoncurrentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration