Long-Term Debt - Summary of Long-Term Debt (Parenthetical) (Detail)	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
5.375% Notes, Due Oct. 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Oct. 31, 2020
Interest rate on debt	5.375%	5.375%
2.40% Term Loan, Due Jun. 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Jun. 30, 2022
Interest rate on debt	2.40%	2.40%
2.05% Term Loan, Due Mar. 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Mar. 31, 2023
Interest rate on debt	2.05%	2.05%
1.30% Term Loan, Due Jun. 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Jun. 30, 2023
Interest rate on debt	1.30%	1.30%
1.55% Term Loan, Due Sep. 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Sep. 30, 2025
Interest rate on debt	1.55%	1.55%
Revolving Credit Facility, Due Mar. 2020 [Member]
Debt Instrument [Line Items]
Long-term debt, maturity date	Mar. 31, 2020

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

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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
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/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

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=glt_FivePointThreeSevenFivePercentNotesDueOctoberTwoThousandTwentyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=glt_TwoPointFourZeroPercentTermLoanDueJuneTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=glt_TwoPointZeroFivePercentTermLoanDueMarchTwoThousandTwentyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=glt_OnePointThreeZeroPercentageTermLoanDueJuneTwoThousandTwentyThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=glt_OnePointFiveFivePercentageTermLoanDueSeptemberTwoThousandTwentyFiveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=glt_RevolvingCreditFacilityDueMarchTwoThousandTwentyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: