Amount of letters of credit issued and reducing availability under revolving credit facility.

No definition available.

Name:	glt_AmountOfLettersOfCreditIssuedAndReducingAvailabilityUnderRevolvingCreditFacility
Namespace Prefix:	glt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt instrument covenant compliance leverage ratio actual.

+ References

No definition available.

+ Details

Name:	glt_DebtInstrumentCovenantComplianceLeverageRatioActual
Namespace Prefix:	glt_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument covenant compliance leverage ratio threshold.

+ References

No definition available.

+ Details

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Namespace Prefix:	glt_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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/disclosureRef
-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=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for amortization of deferred issuance costs associated with an own-share lending arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 2A
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=7656903&loc=SL6757479-112611

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
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- Details

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- Details

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- Details

Name:	us-gaap_VariableRateAxis=glt_EuriborMember
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- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
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Long-Term Debt - Additional Information (Detail)	3 Months Ended	12 Months Ended
Long-Term Debt - Additional Information (Detail)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 01, 2019	Feb. 08, 2019 USD ($)	Feb. 08, 2019 EUR (€)
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity \| €						€ 220,000,000
Interest rate on debt				1.25%
Debt instrument covenant compliance leverage ratio, threshold	4.5	4.0
Debt instrument covenant compliance leverage ratio, actual		1.8
Long-term debt, maturity date		Feb. 08, 2024
Amortization expense related to deferred debt issuance costs		$ 600,000
Other Assets [Member]
Debt Instrument [Line Items]
Unamortized deferred debt issuance costs	$ 1,900,000	$ 1,900,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 400,000,000
Federal fund rate spread		1.00%
Revolving Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		0.125%
Revolving Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		1.00%
Revolving Credit Facility [Member] \| Federal Funds Rate [Member]
Debt Instrument [Line Items]
Federal fund rate spread		0.50%
Revolving Credit Facility [Member] \| Euro Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		1.125%
Revolving Credit Facility [Member] \| Euro Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		2.00%
Revolving Credit Facility [Member] \| EURIBOR [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		112.50%
Revolving Credit Facility [Member] \| EURIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Federal fund rate spread		200.00%
Letters of Credit [Member]
Debt Instrument [Line Items]
Letters of credit issued	7,300,000	$ 7,300,000	$ 7,300,000
Letters of credit outstanding	$ 0	$ 0