Current liability established for restructuring and related charges.

No definition available.

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

- Definition

The number of positions eliminated during the period as a percentage of total positions eliminated during the period in connection with the restructuring plan(s).

+ References

No definition available.

+ Details

Name:	us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminatedPeriodPercent
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_RestructuringCostAndReserveLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after cash payment, of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

Name:	us-gaap_RestructuringCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RestructuringPlanAxis=gti_A2014CorporateAndResearchDevelopmentRationalizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_PredecessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Rationalizations (Details) - USD ($) $ in Millions	3 Months Ended	5 Months Ended	7 Months Ended	12 Months Ended
Rationalizations (Details) - USD ($) $ in Millions	Sep. 30, 2014	Dec. 31, 2015	Aug. 14, 2015	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Rationalizations liability				$ 0.0	$ 0.1
Restructuring costs		$ 0.4		$ 0.7	$ 2.4
2014 Corporate and Research & Development Rationalization [Member]
Restructuring Cost and Reserve [Line Items]
Percent of positions eliminated	25.00%
Predecessor [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring costs			$ 4.4