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Income Taxes (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2015
Dec. 31, 2011
Income Tax Examination [Line Items]          
U.S. federal income tax rate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate    
Net non-current deferred tax assets $ 16,819,000us-gaap_DeferredTaxAssetsNetNoncurrent $ 10,334,000us-gaap_DeferredTaxAssetsNetNoncurrent      
Net non-current deferred tax liabilities 28,197,000us-gaap_DeferredTaxLiabilitiesNoncurrent 41,684,000us-gaap_DeferredTaxLiabilitiesNoncurrent      
Total foreign tax credit carryforwards 64,214,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign 46,283,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign      
Deferred Tax Assets, Operating Loss Carryforwards 25,600,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards        
Unrecognized tax benefits 3,710,000us-gaap_UnrecognizedTaxBenefits 7,203,000us-gaap_UnrecognizedTaxBenefits 9,769,000us-gaap_UnrecognizedTaxBenefits   16,788,000us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits that would have a favorable impact on effective tax rate   2,700,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate      
Accrued interest and penalties 600,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued 500,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued 1,000,000us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued    
Changes attributable to movement in underlying assets 73,400,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount        
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations 1,180,000us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations 0us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations 163,000us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations    
Effective Income Tax Rate Reconciliation, Tax Settlement, Foreign, Amount 1,100,000,000us-gaap_IncomeTaxReconciliationTaxSettlementsForeign        
Prepaid Expenses And Other Current Assets [Member]          
Income Tax Examination [Line Items]          
Net current deferred income tax assets 28,400,000us-gaap_DeferredTaxAssetsNetCurrent
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember		 37,100,000us-gaap_DeferredTaxAssetsNetCurrent
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember		      
Accrued Income And Other Taxes [Member]          
Income Tax Examination [Line Items]          
Net current deferred tax liabilities 7,200,000us-gaap_DeferredTaxLiabilitiesCurrent
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= gti_AccruedIncomeAndOtherTaxesMember		 10,900,000us-gaap_DeferredTaxLiabilitiesCurrent
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= gti_AccruedIncomeAndOtherTaxesMember		      
Foreign Country [Member]          
Income Tax Examination [Line Items]          
Total foreign tax credit carryforwards   19,500,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		      
Tax credit carryforwards, years 7 years        
Operating loss carryforwards   23,100,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		      
Domestic Tax Authority [Member]          
Income Tax Examination [Line Items]          
Deferred Tax Assets, Operating Loss Carryforwards 44,300,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		        
Operating loss carryforwards 36,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		        
State [Member]          
Income Tax Examination [Line Items]          
Operating loss carryforwards 191,500,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		        
Scenario, Forecast [Member]          
Income Tax Examination [Line Items]          
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations       $ 2,000,000us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember