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

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Namespace Prefix:	glt_
Data Type:	xbrli:pureItemType
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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

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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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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

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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

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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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

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
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- Details

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- Details

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Long-Term Debt - Additional Information (Detail)	3 Months Ended
Long-Term Debt - Additional Information (Detail)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2021 EUR (€)	Jul. 01, 2019	Feb. 08, 2019 USD ($)	Feb. 08, 2019 EUR (€)
Debt Instrument [Line Items]
Interest rate on debt				1.25%
Debt instrument covenant compliance leverage ratio, threshold	4.0	4.5
Debt instrument covenant compliance leverage ratio, actual	1.8
Long-term debt, maturity date	Feb. 08, 2024
Term Loans [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity \| €			€ 220,000,000			€ 220,000,000
5.375% Notes [Member]
Debt Instrument [Line Items]
Interest rate on debt	5.375%		5.375%
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	1.125%
Revolving Credit Facility [Member] \| EURIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Federal fund rate spread	2.00%
Letters of Credit [Member]
Debt Instrument [Line Items]
Letters of credit issued	$ 7,300,000	$ 7,300,000
Letters of credit outstanding	$ 0