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Long-Term Debt - Additional Information (Detail) | 0 Months Ended |  | 3 Months Ended |  | 3 Months Ended |  |  | 3 Months Ended |  | 3 Months Ended |  | 3 Months Ended |
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Mar. 31, 2015 | Mar. 31, 2015 | Mar. 31, 2015 Other Assets [Member] USD ($) | Mar. 31, 2015 Maximum [Member] | Mar. 31, 2015 Federal Funds Rate [Member] | Mar. 31, 2015 Revolving Credit Facility [Member] | Mar. 12, 2015 Revolving Credit Facility [Member] USD ($) | Mar. 31, 2015 Revolving Credit Facility, Due Nov. 2016 [Member] | Mar. 31, 2015 Revolving Credit Facility, Due Nov. 2016 [Member] Minimum [Member] | Mar. 31, 2015 Revolving Credit Facility, Due Nov. 2016 [Member] Maximum [Member] | Mar. 31, 2015 Revolving Credit Facility, Due Nov. 2016 [Member] Daily Euro Rate [Member] Minimum [Member] | Mar. 31, 2015 Revolving Credit Facility, Due Nov. 2016 [Member] Daily Euro Rate [Member] Maximum [Member] | Mar. 31, 2015 Letters of Credit [Member] USD ($) | Dec. 31, 2014 Letters of Credit [Member] USD ($) | Mar. 31, 2015 IKB Loan [Member] DebtAgreements Installment | Apr. 11, 2013 IKB Loan [Member] USD ($) | Apr. 11, 2013 IKB Loan [Member] EUR (€) | Mar. 31, 2015 2014 IKB Loan [Member] Installment | Sep. 04, 2014 2014 IKB Loan [Member] USD ($) | Sep. 04, 2014 2014 IKB Loan [Member] EUR (€) | Mar. 31, 2015 5.375% Notes [Member] | Oct. 03, 2012 5.375% Notes [Member] USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  | $ 400,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity date |  |  |  |  |  | Mar. 12, 2020 |  | Nov. 30, 2016 |  |  |  |  |  |  |  |  |  |  |  |  | Oct. 15, 2020 |
Federal fund rate spread |  |  |  |  | 0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_VariableRateAxis = glt_FederalFundsRateMember |  |  |  | 0.125%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_RangeAxis = us-gaap_MinimumMember | 1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_RangeAxis = us-gaap_MaximumMember | 1.125%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_LineOfCreditMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_VariableRateAxis = us-gaap_EurodollarMember | 2.00%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 | 1.9 glt_LeverageRatio |  |  | 3.5 glt_LeverageRatio / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 57,600,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = glt_IkbLoanMember | 42,700,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = glt_IkbLoanMember |  | 12,600,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = glt_IkbLoanTwoThousandFourteenMember | 10,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = glt_IkbLoanTwoThousandFourteenMember |  | 250,000,000 us-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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Quarterly |  |  | 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 glt_DebtInstrumentLoanAgreementNumber / us-gaap_DebtInstrumentAxis = glt_IkbLoanMember |  |  |  |  |  |  |
Number of installments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 32 glt_DebtInstrumentNumberOfInstallmentPayment / us-gaap_DebtInstrumentAxis = glt_IkbLoanMember |  |  | 27 glt_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 |  |  | 6,100,000 us-gaap_UnamortizedDebtIssuanceExpense / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Letters of credit issued |  |  |  |  |  |  |  |  |  |  |  |  | 5,400,000 glt_AmountOfLettersOfCreditIssuedAndReducingAvailabilityUnderRevolvingCreditFacility / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember | 5,300,000 glt_AmountOfLettersOfCreditIssuedAndReducingAvailabilityUnderRevolvingCreditFacility / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember |  |  |  |  |  |  |  |
Letters of credit outstanding |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember |  |  |  |  |  |  |  |  |

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- Definition Amount of Letters of Credit Issued and Reducing Availability under Revolving Credit Facility + References No definition available. + Details
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- Definition Debt Instrument Loan Agreement Number + References No definition available. + Details
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- Definition Debt instrument number of installment payment. + References No definition available. + Details
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- Definition Ratio of EBITDA over net debt. + References No definition available. + Details
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- Definition Loan repayable installments end date. + References No definition available. + Details
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- Definition Loan repayable installments start date. + References No definition available. + Details
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- Definition Margin over Euro-rate. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
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- Definition Description of the frequency of periodic payments (monthly, quarterly, annual). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Details
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- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Description of debt redemption features under terms of the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph e -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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