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

X

- 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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X

- 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

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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

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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

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