Borrowing Threshold

No definition available.

Name:	gti_BorrowingThreshold
Namespace Prefix:	gti_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Borrowing Threshold Percentage

+ References

No definition available.

+ Details

Name:	gti_BorrowingThresholdPercentage
Namespace Prefix:	gti_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Amortization Rate

+ References

No definition available.

+ Details

Name:	gti_DebtInstrumentAmortizationRate
Namespace Prefix:	gti_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument Prepayment Premium

+ References

No definition available.

+ Details

Name:	gti_DebtInstrumentPrepaymentPremium
Namespace Prefix:	gti_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Equity Interest Pledge

+ References

No definition available.

+ Details

Name:	gti_EquityInterestPledge
Namespace Prefix:	gti_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Excess Cashflow Threshold Percentage

+ References

No definition available.

+ Details

Name:	gti_ExcessCashflowThresholdPercentage
Namespace Prefix:	gti_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Minimum liquidity required by credit facility

+ References

No definition available.

+ Details

Name:	gti_MinimumLiquidity
Namespace Prefix:	gti_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph e
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentRedemptionPricePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: 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(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

Name:	us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
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- Details

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- Details

Name:	us-gaap_VariableRateAxis=gti_ABRMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gti_A2018TermLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
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Balance Type:
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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Subsequent events - Refinancing (Details)		6 Months Ended
Subsequent events - Refinancing (Details)	Feb. 12, 2018 USD ($)	Jun. 30, 2018	Jun. 15, 2018 USD ($)	Mar. 31, 2018 USD ($)	Apr. 27, 2016 USD ($)
Subsequent Event
Minimum Liquidity					$ 25,000,000
Equity interest pledge				65.00%
Senior Notes
Subsequent Event
Debt Instrument, Redemption Price, Percentage		101.594%
2018 Credit Agreement
Subsequent Event
Leverage ratio				4.00
Revolving Credit Facility \| 2018 Revolving Credit Facility
Subsequent Event
Borrowing capacity	$ 250,000,000		$ 2,250,000,000	$ 1,500,000,000
Borrowing threshold (greater than)				$ 35,000,000
Borrowing threshold percentage				35.00%
Revolving Credit Facility \| 2018 Revolving Credit Facility \| LIBO
Subsequent Event
Margin spread on variable interest rate	3.75%
Revolving Credit Facility \| 2018 Revolving Credit Facility \| ABR
Subsequent Event
Margin spread on variable interest rate	2.75%
Line of Credit \| 2018 Term Loan Facility
Subsequent Event
Borrowing capacity	$ 1,500,000,000
Margin spread on variable interest rate		0.25%
Debt Instrument, Amortization Rate				5.00%
Prepayment premium				1.00%
Excess Cashflow Threshold Percentage				75.00%
Line of Credit \| 2018 Term Loan Facility \| Contingent Event One
Subsequent Event
Excess Cashflow Threshold Percentage				50.00%
Line of Credit \| 2018 Term Loan Facility \| Contingent Event Two
Subsequent Event
Excess Cashflow Threshold Percentage				0.00%
Leverage ratio				1.25
Line of Credit \| 2018 Term Loan Facility \| Minimum \| Contingent Event One
Subsequent Event
Leverage ratio				1.25
Line of Credit \| 2018 Term Loan Facility \| Maximum \| Contingent Event One
Subsequent Event
Leverage ratio				1.75
Line of Credit \| 2018 Term Loan Facility \| LIBO
Subsequent Event
Margin spread on variable interest rate	3.50%
Undrawn commitment fee	0.25%
Line of Credit \| 2018 Term Loan Facility \| ABR
Subsequent Event
Margin spread on variable interest rate	2.50%
Subsequent Event
Subsequent Event
Equity interest pledge	65.00%
Subsequent Event \| Senior Notes
Subsequent Event
Debt Instrument, Redemption Price, Percentage	101.594%
Subsequent Event \| 2018 Credit Agreement
Subsequent Event
Leverage ratio	4.00
Subsequent Event \| Revolving Credit Facility \| 2018 Revolving Credit Facility
Subsequent Event
Borrowing capacity	$ 250,000,000
Undrawn commitment fee	0.25%
Borrowing threshold (greater than)	$ 35,000,000
Borrowing threshold percentage	35.00%
Subsequent Event \| Revolving Credit Facility \| 2018 Revolving Credit Facility \| LIBO
Subsequent Event
Margin spread on variable interest rate	3.75%
Subsequent Event \| Revolving Credit Facility \| 2018 Revolving Credit Facility \| ABR
Subsequent Event
Margin spread on variable interest rate	2.75%
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility
Subsequent Event
Borrowing capacity	$ 1,500,000,000
Margin spread on variable interest rate	0.25%
Debt Instrument, Amortization Rate	5.00%
Prepayment premium	1.00%
Excess Cashflow Threshold Percentage	75.00%
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| Contingent Event One
Subsequent Event
Excess Cashflow Threshold Percentage	50.00%
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| Contingent Event Two
Subsequent Event
Excess Cashflow Threshold Percentage	0.00%
Leverage ratio	1.25
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| Minimum \| Contingent Event One
Subsequent Event
Leverage ratio	1.25
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| Maximum \| Contingent Event One
Subsequent Event
Leverage ratio	1.75
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| LIBO
Subsequent Event
Margin spread on variable interest rate	3.50%
Subsequent Event \| Line of Credit \| 2018 Term Loan Facility \| ABR
Subsequent Event
Margin spread on variable interest rate	2.50%