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Supply Chain Financing (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Supply Chain Financing [Abstract] |  |  |
Maximum amount owing to financing party | $ 49,300,000 gti_MaximumAmountOwingToFinancingParty |  |
Supply chain financing liability | 0 us-gaap_OtherShortTermBorrowings | 9,455,000 us-gaap_OtherShortTermBorrowings |
Supply Chain Financing Mark Up | $ 47,000 gti_SupplyChainFinancingMarkUp | $ 451,000 gti_SupplyChainFinancingMarkUp | $ 621,000 gti_SupplyChainFinancingMarkUp

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- Definition Maximum amount owing to financing party. + References No definition available. + Details
Name: | gti_MaximumAmountOwingToFinancingParty
Namespace Prefix: | gti_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | gti_SupplyChainFinancingAbstract
Namespace Prefix: | gti_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Supply Chain Financing Mark Up + References No definition available. + Details
Name: | gti_SupplyChainFinancingMarkUp
Namespace Prefix: | gti_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Sum of the carrying amounts at the balance sheet date of short-term borrowings not otherwise specified in the taxonomy having initial terms less than one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)(7)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_OtherShortTermBorrowings
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant