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Income Taxes - Schedule of Reconciliation of Federal Statutory Rate to Company's Effective Tax Rate (Detail)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Federal income tax provision at statutory rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal income tax benefit 0.20%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 0.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 1.30%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Foreign income tax rate differential (5.00%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (5.00%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (3.90%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential
Change in statutory tax rates (2.20%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate (0.60%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate (0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate
Tax credits (2.00%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (4.40%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits (0.50%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits
Change in unrecognized tax benefits, net 1.30%glt_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefitsNet (22.70%)glt_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefitsNet 0.40%glt_EffectiveIncomeTaxRateReconciliationChangeInUnrecognizedTaxBenefitsNet
Permanent differences on non-U.S. earnings (2.80%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlementsForeign (0.40%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlementsForeign  
Cellulosic biofuel credit, net of incremental state tax and manufacturing deduction benefit     (6.10%)glt_EffectiveIncomeTaxRateReconciliationTaxCreditsNetOfIncrementalStateTaxAndManufacturingDeductionBenefit
Valuation allowance (2.70%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance    
Other (1.00%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.60%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Provision for income taxes 20.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations 3.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 24.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations