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Financial Derivatives and Hedging Activities - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2018
Maximum [Member] | Fair Value, Measurements [Member] |
Derivative [Line Items] |
Accumulated other comprehensive income realization period | 18 months
Minimum [Member] | Fair Value, Measurements [Member] |
Derivative [Line Items] |
Accumulated other comprehensive income realization period | 12 months
Designated as Hedging [Member] | Foreign Exchange [Member] |
Derivative [Line Items] |
Maturities of foreign currency derivative contracts | 18 months
Designated as Hedging [Member] | Cash Flow Hedges [Member] | Maximum [Member] |
Derivative [Line Items] |
Period of production costs expected to be incurred | 18 months
Not Designated as Hedging [Member] |
Derivative [Line Items] |
Maturities of foreign currency derivative contracts | 1 month

X
- Definition Period over which deferred gains or losses are expected to be reclassified out of accumulated other comprehensive income. + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maturities of foreign currency derivative contracts. + References No definition available. + Details
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Namespace Prefix: | glt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period over which hedged production cost are expected to be incurred. + References No definition available. + Details
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Namespace Prefix: | glt_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |