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Leases - Additional Information (Details) - USD ($) $ in Thousands | May 13, 2021 | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | --- | ---
Operating Leased Assets [Line Items] |  |  |
Right of use asset |  | $ 24,865 | $ 11,789
Mount Holly Acquisition [Member] |  |  |
Operating Leased Assets [Line Items] |  |  |
Lease obligation expiry | Jul. 31, 2077 |  |
Right of use asset | $ 7,700 |  |

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- Definition Date which lease or group of leases is set to expire, in CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_LeaseExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
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
Name: | us-gaap_OperatingLeasedAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=glt_MountHollyAcquisitionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |