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Gains on Dispositions of Plant, Equipment and Timberlands, Net - Summary for Sale of Timberlands and Other Assets (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013
Property, Plant and Equipment [Line Items] |  |  |  |
Proceeds |  |  | $ 2,360 | $ 92
Gain | 1,482 | 19 | 2,291 | 92
Timberlands [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Acres |  |  | 935 |
Proceeds |  |  | 2,355 |
Gain |  |  | 2,290 |
Other [Member] |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |
Proceeds |  |  | 5 | 92
Gain |  |  | $ 1 | $ 92

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- Definition Acres of timberland. + References No definition available. + Details
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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
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Balance Type: | credit
Period Type: | duration

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- Details
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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
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | duration