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Supplementary Balance Sheet Detail (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]    
Cost in excess of normal absorption $ 0gti_InventoryCostsExcessOfNormalAbsorption $ 3.5gti_InventoryCostsExcessOfNormalAbsorption
Rationalizations [Member]    
Restructuring Cost and Reserve [Line Items]    
Additions $ 19.6gti_InventoryValuationReservesAdditions
/ us-gaap_IncomeStatementLocationAxis
= gti_RationalizationsMember		 $ 9.1gti_InventoryValuationReservesAdditions
/ us-gaap_IncomeStatementLocationAxis
= gti_RationalizationsMember