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Asset Retirement Obligation - Schedule of Consolidated Balance Sheets where Asset Retirement Obligations Recorded (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Asset retirement obligations $ 3,617us-gaap_AssetRetirementObligation $ 4,114us-gaap_AssetRetirementObligation $ 4,851us-gaap_AssetRetirementObligation $ 5,032us-gaap_AssetRetirementObligation
Other Current Liabilities [Member]        
Asset retirement obligations 2,000us-gaap_AssetRetirementObligation
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember		 2,855us-gaap_AssetRetirementObligation
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherCurrentLiabilitiesMember		    
Other Long-Term Liabilities [Member]        
Asset retirement obligations $ 1,617us-gaap_AssetRetirementObligation
/ us-gaap_BalanceSheetLocationAxis
= glt_OtherLongTermLiabilitiesMember		 $ 1,259us-gaap_AssetRetirementObligation
/ us-gaap_BalanceSheetLocationAxis
= glt_OtherLongTermLiabilitiesMember