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Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Derivative [Line Items] |  |  |
Intercompany debt designated as a non-derivative net investment hedging instrument | $ 15.1 us-gaap_NetInvestmentHedgeDerivativesAtFairValueNet |  | $ 15.8 us-gaap_NetInvestmentHedgeDerivativesAtFairValueNet
Gain (loss) currency translation adjustment | 0.7 us-gaap_DerivativeNetHedgeIneffectivenessGainLoss | 0.1 us-gaap_DerivativeNetHedgeIneffectivenessGainLoss |
Foreign Currency Derivatives |  |  |
Derivative [Line Items] |  |  |
Notional amounts of foreign currency derivatives |  |  | 108.7 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = gti_ForeignCurrencyDerivativesMember
Refined Oil Products | Commodity Derivative Contracts |  |  |
Derivative [Line Items] |  |  |
Outstanding derivative swap contracts | $ 4.4 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = gti_CommodityDerivativeContractsMember / us-gaap_TradingActivityByTypeAxis = gti_RefinedOilProductsMember |  |
Minimum | Foreign Currency Derivatives |  |  |
Derivative [Line Items] |  |  |
Derivative maturities | April 2015 |  |
Maximum | Foreign Currency Derivatives |  |  |
Derivative [Line Items] |  |  |
Derivative maturities | December 2015 |  |
Maximum | Commodity Derivative Contracts |  |  |
Derivative [Line Items] |  |  |
Derivative maturities | April 2015 |  |

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- Definition Derivative maturities + References No definition available. + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Nominal or face amount used to calculate payments on the derivative liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Notional Amount -URI http://asc.fasb.org/extlink&oid=6519104 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959 + Details
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- Details
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- Definition Amount of gain (loss) from the ineffectiveness of the hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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- Definition Net fair value of all derivatives designated as hedges of net investment in foreign operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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