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

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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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://fasb.org/us-gaap/role/ref/legacyRef
-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

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15)(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15)(5))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- 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

+ Details

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

- 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

Percentage of tier one capital to total leverage exposure, as defined by regulatory framework.

+ References

No definition available.

+ Details

Name:	us-gaap_SupplementaryLeverageRatio
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentAxis=gti_A2018CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gti_A2018RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gti_ContingentEventAxis=gti_ContingentEventOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gti_ContingentEventAxis=gti_ContingentEventTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=gti_BrookfieldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gti_BrookfieldPromissoryNoteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gti_AnniversaryAxis=gti_ThirdAnniversaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt And Liquidity (Refinancing) (Details) - USD ($)			12 Months Ended
Debt And Liquidity (Refinancing) (Details) - USD ($)	Apr. 19, 2018	Feb. 12, 2018	Dec. 31, 2019	Jun. 15, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Equity Interest Pledge		65.00%
Dividends Payable	$ 160,000,000
2018 Credit Agreement [Member]
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital		4.00
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage			101.594%
Line of Credit [Member] \| 2018 Term Loan Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,500,000,000
Debt Instrument, Amortization Rate		5.00%
Debt Instrument Prepayment Premium		1.00%
Excess Cashflow Threshold Percentage		75.00%
Line of Credit [Member] \| 2018 Term Loan Facility [Member] \| LIBO [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		3.50%
Line of Credit [Member] \| 2018 Term Loan Facility [Member] \| ABR [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.50%
Revolving Credit Facility [Member] \| 2018 Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 250,000,000		$ 2,250,000,000	$ 1,500,000,000
Borrowing Threshold		$ 35,000,000
Borrowing Threshold Percentage		35.00%
Revolving Credit Facility [Member] \| 2018 Revolving Credit Facility [Member] \| LIBO [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		3.75%
Revolving Credit Facility [Member] \| 2018 Revolving Credit Facility [Member] \| ABR [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.75%
Contingent Event One [Member] \| Line of Credit [Member] \| 2018 Term Loan Facility [Member]
Debt Instrument [Line Items]
Excess Cashflow Threshold Percentage		50.00%
Contingent Event One [Member] \| Line of Credit [Member] \| 2018 Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital		1.25
Contingent Event One [Member] \| Line of Credit [Member] \| 2018 Term Loan Facility [Member] \| Maximum
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital		1.75
Contingent Event Two [Member] \| Line of Credit [Member] \| 2018 Term Loan Facility [Member]
Debt Instrument [Line Items]
Excess Cashflow Threshold Percentage		0.00%
Brookfield [Member] \| Brookfield Promissory Note [Member]
Debt Instrument [Line Items]
Dividends Payable	$ 750,000,000
Brookfield [Member] \| Brookfield Promissory Note [Member] \| Maximum
Debt Instrument [Line Items]
Supplementary Leverage Ratio	175.00%
Brookfield [Member] \| Brookfield Promissory Note [Member] \| LIBO [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	4.50%
Third Anniversary [Member] \| Brookfield [Member] \| Brookfield Promissory Note [Member] \| LIBO [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%