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Income Taxes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Contingency [Line Items]        
Deferred taxes and other included a deferred tax benefit $ 9,600,000us-gaap_DeferredOtherTaxExpenseBenefit $ 15,100,000us-gaap_DeferredOtherTaxExpenseBenefit $ 2,300,000us-gaap_DeferredOtherTaxExpenseBenefit  
Other taxes total expense 200,000us-gaap_OtherTaxExpenseBenefit 3,600,000us-gaap_OtherTaxExpenseBenefit 100,000us-gaap_OtherTaxExpenseBenefit  
Federal income tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Valuation allowance 3,200,000us-gaap_DeferredTaxAssetsValuationAllowance 6,200,000us-gaap_DeferredTaxAssetsValuationAllowance    
Research and development, tax credit 1,800,000glt_ResearchAndDevelopmentTaxCredits 3,000,000glt_ResearchAndDevelopmentTaxCredits 400,000glt_ResearchAndDevelopmentTaxCredits  
Unremitted earnings, reinvested 305,600,000us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings 288,800,000us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings    
Deferred tax liability 0us-gaap_DeferredTaxLiabilities      
Gross unrecognized tax benefits 14,900,000us-gaap_UnrecognizedTaxBenefits 14,900,000us-gaap_UnrecognizedTaxBenefits 30,400,000us-gaap_UnrecognizedTaxBenefits 29,700,000us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits that would impact effective tax rate 14,900,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate      
Lower range of gross unrecognized tax benefits balance may decrease within the next twelve months 0us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeLowerBound      
Higher range of gross unrecognized tax benefits balance may decrease within the next twelve months 5,200,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleEstimatedRangeOfChangeUpperBound      
United States - State [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards 86,900,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		      
Tax credit carryforwards 400,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		      
United States - State [Member] | Minimum [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards expiration year 2015      
Tax credit carryforwards expiration year 2015      
United States - State [Member] | Maximum [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards expiration year 2031      
Tax credit carryforwards expiration year 2027      
Foreign Tax Authority [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards 24,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		      
Tax credit carryforwards $ 3,300,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		      
Foreign Tax Authority [Member] | Minimum [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards expiration year 2019      
Tax credit carryforwards expiration year 2020      
Foreign Tax Authority [Member] | Maximum [Member]        
Income Tax Contingency [Line Items]        
Net operating loss carryforwards expiration year 2033      
Tax credit carryforwards expiration year 2034