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Retirement Plans and Other Post-Retirement Benefits - Benefit Payments Expected to be Made under Non-Qualified Pension Plans and Other Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Nonqualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit payments expected	$ 2,108glt_DefinedBenefitsPlanExpectedBenefitPayments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = glt_NonQualifiedPensionPlanMember
Other Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit payments expected	$ 3,769glt_DefinedBenefitsPlanExpectedBenefitPayments / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = glt_OtherDefinedBenefitPlanMember

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- Definition

Defined benefits plan expected benefit payments.

+ References

No definition available.

+ Details

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

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- Details

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

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- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=glt_NonQualifiedPensionPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=glt_OtherDefinedBenefitPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: