12 Month Trailing EBITDA

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

12 Month Trailing EBITDA Minimum (High End)

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Minimum liquidity required by credit facility

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Repurchase Percentage Price Of Aggregate Principal Due To Change In Control

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Term loan portion of revolving credit facility balance

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Unpaid Indebtness After Maturity Or Acceleration

+ References

No definition available.

+ Details

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Namespace Prefix:	gti_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

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-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

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Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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(22))
-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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

+ Details

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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

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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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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Balance Type:
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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 (Old Revolving Facility and Term Loan Facility) (Details) - USD ($)	Feb. 27, 2015	Dec. 31, 2018	Dec. 31, 2017	Apr. 27, 2016	Aug. 11, 2015	Jul. 28, 2015	Apr. 23, 2014	Nov. 20, 2012
Debt Instrument [Line Items]
Minimum Liquidity				$ 25,000,000
12 Month Trailing EBITDA			$ 75,000,000
12 Month Trailing EBITDA Minimum (High End)				35,000,000
Term Loan Balance					$ 40,000,000
Debt Instrument, Interest Rate, Stated Percentage		6.375%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		$ 0	58,192,000
Letters of Credit Outstanding, Amount			48,200,000
Revolving Credit Facility [Member] \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity							$ 400,000,000
Revolving Credit Facility [Member] \| Amended and Restated Credit Agreement July 2015 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 225,000,000		$ 375,000,000
Long-term Line of Credit			39,500,000
Letters of Credit Outstanding, Amount			8,700,000
Term Loan Facility [Member] \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 40,000,000
Minimum [Member] \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Minimum [Member] \| London Interbank Offered Rate (LIBOR) [Member] \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Maximum \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.70%
Maximum \| London Interbank Offered Rate (LIBOR) [Member] \| Amended and Restated Credit Agreement February 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	4.75%
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 300,000,000
Debt Instrument, Interest Rate, Stated Percentage								6.375%
Repurchase Percentage Price Of Aggregate Principal Due To Change In Control								101.00%
Senior Notes [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Unpaid Indebtness After Maturity Or Acceleration			$ 50,000,000.0