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Segment Reporting Revenue from External Customers by Products and Services (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Revenue from External Customer [Line Items]      
Revenue, Net $ 1,085,304us-gaap_SalesRevenueNet $ 1,166,674us-gaap_SalesRevenueNet $ 1,248,264us-gaap_SalesRevenueNet
UNITED STATES      
Revenue from External Customer [Line Items]      
Revenue, Net 284,209us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= country_US		 289,866us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= country_US		 372,014us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= country_US
Americas [Member]      
Revenue from External Customer [Line Items]      
Revenue, Net 180,070us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AmericasMember		 177,602us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AmericasMember		 195,748us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AmericasMember
Asia Pacific [Member]      
Revenue from External Customer [Line Items]      
Revenue, Net 192,230us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaPacificMember		 220,945us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaPacificMember		 235,658us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= us-gaap_AsiaPacificMember
Europe Middle East Africa [Member]      
Revenue from External Customer [Line Items]      
Revenue, Net $ 428,795us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= gti_EuropeMiddleEastAfricaMember		 $ 478,261us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= gti_EuropeMiddleEastAfricaMember		 $ 444,844us-gaap_SalesRevenueNet
/ us-gaap_StatementGeographicalAxis
= gti_EuropeMiddleEastAfricaMember