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Gain on Dispositions of Plant, Equipment and Timberlands - Summary for Sale of Timberlands and Other Assets (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Property, Plant and Equipment [Line Items] |  |  |
Proceeds | $ 5,072 us-gaap_TimberRevenue | $ 1,947 us-gaap_TimberRevenue | $ 10,272 us-gaap_TimberRevenue
Gain | 4,861 us-gaap_GainLossOnSaleOfPropertyPlantEquipment | 1,726 us-gaap_GainLossOnSaleOfPropertyPlantEquipment | 9,815 us-gaap_GainLossOnSaleOfPropertyPlantEquipment
Timberlands [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Acres | 2,753 glt_AcresOfTimberland / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 876 glt_AcresOfTimberland / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 4,830 glt_AcresOfTimberland / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember
Proceeds | 5,062 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 1,445 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 9,494 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember
Gain | 4,855 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 1,410 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember | 9,203 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = glt_TimberlandsMember
Other [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Proceeds | 10 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember | 502 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember | 778 us-gaap_TimberRevenue / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
Gain | $ 6 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember | $ 316 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember | $ 612 us-gaap_GainLossOnSaleOfPropertyPlantEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember

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- Definition Acres of timberland. + References No definition available. + Details
Name: | glt_AcresOfTimberland
Namespace Prefix: | glt_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
Name: | us-gaap_GainLossOnSaleOfPropertyPlantEquipment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Revenue from sale of timber, mill lumber, wood, and other wood products. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(a)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
Name: | us-gaap_TimberRevenue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=glt_TimberlandsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |