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v3.20.2

Restructuring - Additional Information (Details) | 6 Months Ended
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Jun. 30, 2020
Restructuring And Related Activities [Abstract] |
Severance and benefit continuation costs, payment description | The severance and benefit continuation costs are primarily expected to be paid out ratably over the next 6 months.

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- Definition Severance and benefit continuation costs payment description. + References No definition available. + Details
Name: | glt_SeveranceAndBenefitContinuationCostsPaymentDescription
Namespace Prefix: | glt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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