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Long-Term Debt - Additional Information (Detail)
 0 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2014
USD ($)
Dec. 31, 2014
IKB Loan [Member]
DebtAgreements
Installment
Apr. 11, 2013
IKB Loan [Member]
USD ($)
Apr. 11, 2013
IKB Loan [Member]
EUR (€)
Dec. 31, 2014
2014 IKB Loan [Member]
Installment
Sep. 04, 2014
2014 IKB Loan [Member]
USD ($)
Sep. 04, 2014
2014 IKB Loan [Member]
EUR (€)
Dec. 31, 2014
Maximum [Member]
Dec. 31, 2014
Federal Funds Rate [Member]
Dec. 31, 2014
Other Assets [Member]
USD ($)
Dec. 31, 2014
5.375% Notes [Member]
Oct. 03, 2012
5.375% Notes [Member]
USD ($)
Dec. 31, 2014
Revolving Credit Facility [Member]
Nov. 21, 2011
Revolving Credit Facility [Member]
USD ($)
Dec. 31, 2014
Revolving Credit Facility, Due Nov. 2016 [Member]
Dec. 31, 2014
Revolving Credit Facility, Due Nov. 2016 [Member]
Minimum [Member]
Dec. 31, 2014
Revolving Credit Facility, Due Nov. 2016 [Member]
Maximum [Member]
Dec. 31, 2014
Revolving Credit Facility, Due Nov. 2016 [Member]
Daily Euro Rate [Member]
Minimum [Member]
Dec. 31, 2014
Revolving Credit Facility, Due Nov. 2016 [Member]
Daily Euro Rate [Member]
Maximum [Member]
Dec. 31, 2014
Letters of Credit [Member]
USD ($)
Dec. 31, 2013
Letters of Credit [Member]
USD ($)
Debt Instrument [Line Items]                                            
Line of credit facility, maximum borrowing capacity                             $ 350,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember		              
Debt instrument maturity date                       Oct. 15, 2020   Nov. 21, 2016   Nov. 30, 2016            
Federal fund rate spread                   0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableRateAxis
= glt_FederalFundsRateMember		             0.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_VariableRateAxis
= us-gaap_EurodollarMember		 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_CreditFacilityAxis
= us-gaap_LineOfCreditMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_EurodollarMember		    
Margin over Euro-rate   1.00%glt_MarginOverEuroRate                                        
Leverage ratio 2.15glt_LeverageRatio               3.50glt_LeverageRatio
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		                          
Aggregate principal amount       57,600,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanMember		 42,700,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanMember		   12,600,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanTwoThousandFourteenMember		 10,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanTwoThousandFourteenMember		         250,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_LongtermDebtTypeAxis
= glt_FivePointThreeSevenFivePercentNotesMember		                  
Percentage of aggregate principal amount of outstanding     2.05%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanMember		     2.40%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanTwoThousandFourteenMember		           5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= glt_FivePointThreeSevenFivePercentNotesMember		 5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= glt_FivePointThreeSevenFivePercentNotesMember		                  
Frequency of interest payable                       Semiannually                    
Debt instrument redemption                       The 5.375% Notes are redeemable, in whole or in part, at anytime on or after October 15, 2016 at the redemption prices specified in the applicable Indenture. Prior to October 15, 2016, we may redeem some or all of the Notes at a "make-whole" premium as specified in the Indenture.                    
Number of separate loan agreement     2glt_DebtInstrumentLoanAgreementNumber
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanMember		                                      
Number of installments     32glt_DebtInstrumentNumberOfInstallmentPayment
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanMember		     27glt_DebtInstrumentNumberOfInstallmentPayment
/ us-gaap_DebtInstrumentAxis
= glt_IkbLoanTwoThousandFourteenMember		                                
Installments beginning date     Jun. 30, 2015     Sep. 30, 2015                                
Installments ending date     Mar. 31, 2023     Jun. 30, 2022                                
Aggregated unamortized deferred debt issuance costs                     5,100,000us-gaap_UnamortizedDebtIssuanceExpense
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember		                      
Amortization expense related to deferred debt issuance costs   1,300,000us-gaap_DeferredFinanceCostsOwnshareLendingArrangementIssuanceCostsAmortizationExpense                                        
Letters of credit issued                                         5,300,000glt_AmountOfLettersOfCreditIssuedAndReducingAvailabilityUnderRevolvingCreditFacility
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember		 5,200,000glt_AmountOfLettersOfCreditIssuedAndReducingAvailabilityUnderRevolvingCreditFacility
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember
Letters of credit outstanding                                         $ 0us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_CreditFacilityAxis
= us-gaap_LetterOfCreditMember