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Stock-Based Compensation (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 1.6us-gaap_AllocatedShareBasedCompensationExpense $ 0.5us-gaap_AllocatedShareBasedCompensationExpense  
Compensation cost related to non-vested restricted stock, performance shares and stock options not yet recognized     10.6us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Compensation cost related to non-vested restricted stock, performance shares and stock options not yet recognized, weighted average life, in years 1 year 9 months    
Selling And Administrative Expenses [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 1.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= gti_SellingAndAdministrativeExpensesMember
$ 0.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= gti_SellingAndAdministrativeExpensesMember