Additional basic contribution to savings plan by employees in percentage.

No definition available.

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- Definition

Basic contribution to savings plan by employees in percentage.

+ References

No definition available.

+ Details

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- Definition

Defined benefit plan option for employees to remain for maximum additional period.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Employee retirement age.

+ References

No definition available.

+ Details

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Data Type:	xbrli:positiveIntegerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of equity in GTI.

+ References

No definition available.

+ Details

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

- Definition

Percentage of increase in salaries based on the final average wages.

+ References

No definition available.

+ Details

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

- Definition

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount of payment for pension and other postretirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

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Period Type:	duration

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Data Type:	na
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- Details

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Data Type:	na
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- Details

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Data Type:	na
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Retirement plans and postretirement benefits (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Y	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Percentage of increase in salaries based on the final average wages	6.00%
Percentage of equity in GTI	50.00%
Option to remain in defined benefit plan for an additional period, in years	5 years
Defined benefit plan, accumulated benefit obligation	$ 158,600	$ 157,000
Employee retirement age, years \| Y	65
Basic contribution to savings plan by employees in percentage	5.00%
Additional basic contribution to savings plan by employees in percentage	45.00%
Pension and other postretirement benefit contributions	$ 1,600	2,500
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment plan percentage	100.00%
U.S.
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 109,845	100,905	$ 93,897
U.S. \| Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment plan percentage	20.00%
U.S. \| Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment plan percentage	80.00%
Foreign
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 13,618	$ 11,871	$ 11,293