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Stock-Based Compensation - Compensation Expense for Periods (Detail) (Restricted Stock Units (RSU) and Performance Share Awards (PSAs) [Member], USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Restricted Stock Units (RSU) and Performance Share Awards (PSAs) [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense $ 2,652us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= glt_RestrictedStockUnitsAndPerformanceShareAwardsMember
$ 2,882us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= glt_RestrictedStockUnitsAndPerformanceShareAwardsMember
$ 2,576us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= glt_RestrictedStockUnitsAndPerformanceShareAwardsMember