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Supplementary Balance Sheet Detail (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Restructuring Cost and Reserve [Line Items] |  |
Cost in excess of normal absorption | $ 0 gti_InventoryCostsExcessOfNormalAbsorption | $ 3.5 gti_InventoryCostsExcessOfNormalAbsorption
Rationalizations [Member] |  |
Restructuring Cost and Reserve [Line Items] |  |
Additions | $ 19.6 gti_InventoryValuationReservesAdditions / us-gaap_IncomeStatementLocationAxis = gti_RationalizationsMember | $ 9.1 gti_InventoryValuationReservesAdditions / us-gaap_IncomeStatementLocationAxis = gti_RationalizationsMember

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- Definition Inventory costs excess of normal absorption. + References No definition available. + Details
Name: | gti_InventoryCostsExcessOfNormalAbsorption
Namespace Prefix: | gti_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Inventory valuation reserves additions. + References No definition available. + Details
Name: | gti_InventoryValuationReservesAdditions
Namespace Prefix: | gti_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RestructuringCostAndReserveLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=gti_RationalizationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |