Borrowing Threshold

No definition available.

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Balance Type:	credit
Period Type:	instant

- Definition

Borrowing Threshold Percentage

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Amortization Rate

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument Prepayment Premium

+ References

No definition available.

+ Details

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

- Definition

Equity Interest Pledge

+ References

No definition available.

+ Details

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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

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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

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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))
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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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

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
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

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

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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Period Type:	duration

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Debt And Liquidity (Narrative) (Details) - USD ($)				3 Months Ended
Debt And Liquidity (Narrative) (Details) - USD ($)	Dec. 20, 2019	Feb. 13, 2019	Feb. 12, 2018	Mar. 31, 2020	Mar. 31, 2019	Jun. 15, 2018	Apr. 19, 2018	Mar. 31, 2018
Debt Instrument [Line Items]
Repayments of Long-term Debt				$ 0	$ 125,000,000
Equity Interest Pledge			65.00%
Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage				101.594%
2018 Term Loan Facility \| Senior Subordinated Notes
Debt Instrument [Line Items]
Repayments of Long-term Debt	$ 225,000,000	$ 125,000,000
2018 Credit Agreement
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital			4.00
Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 1,500,000,000
Debt Instrument, Basis Spread on Variable Rate				0.25%
Debt Instrument, Amortization Rate			5.00%
Debt Instrument Prepayment Premium			1.00%
Excess Cashflow Threshold Percentage			75.00%
Revolving Credit Facility \| 2018 Revolving Credit Facility
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%
LIBO \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			3.50%
LIBO \| Revolving Credit Facility \| 2018 Revolving Credit Facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			3.75%
ABR \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			2.50%
ABR \| Revolving Credit Facility \| 2018 Revolving Credit Facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			2.75%
Contingent Event One \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Excess Cashflow Threshold Percentage			50.00%
Contingent Event Two \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Excess Cashflow Threshold Percentage			0.00%
Minimum \| Contingent Event One \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital			1.25
Maximum \| Contingent Event One \| Line of Credit \| 2018 Term Loan Facility
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital			1.75
Brookfield \| Brookfield Promissory Note
Debt Instrument [Line Items]
Dividends Payable							$ 750,000,000
Fair Value, Inputs, Level 3
Debt Instrument [Line Items]
Long-term Debt, Fair Value				$ 1,605,000,000