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Supplementary Balance Sheet Detail (Narrative) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
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Balance Sheet Related Disclosures [Abstract] |  |
Cost in excess of normal absorption | $ 0 | $ 3.5

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