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v3.20.1

Retirement Plans and Other Post-Retirement Benefits - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 | Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items] |  |
Employer contribution of employee’s eligible earnings, percentage | 10.00% |
Pension Benefits [Member] | Scenario, Forecast [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pension assets expected to revert |  | $ 33.0
Pension Benefits [Member] | Cash and Cash Equivalents [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Assets remained in pension trust | $ 51.1 |

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- Definition Defined benefit plan pension assets expected to revert net of excise tax. + References No definition available. + Details
Name: | glt_DefinedBenefitPlanPensionAssetsExpectedToRevertNetOfExciseTax
Namespace Prefix: | glt_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Excess plan assets remaining after plan termination, before excise tax. + References No definition available. + Details
Name: | glt_DefinedBenefitPlanPensionAssetsTransferredFromPlanTrust
Namespace Prefix: | glt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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