Maximum aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

No definition available.

Name:	gnrt_MaximumDerivativeNotionalAmount
Namespace Prefix:	gnrt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of interest rate swap agreements with payment dates modified.

+ References

No definition available.

+ Details

Name:	gnrt_NumberOfInterestRateSwapsWithModifiedPaymentDates
Namespace Prefix:	gnrt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of remaining interest rate swap agreements with payment dates modified.

+ References

No definition available.

+ Details

Name:	gnrt_NumberOfRemainingInterestRateSwapsWithModifiedPaymentDates
Namespace Prefix:	gnrt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of hedge effectiveness.

+ References

No definition available.

+ Details

Name:	gnrt_PercentageOfEffectivenessOnNetInvestmentHedges
Namespace Prefix:	gnrt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the approximate percentage of fixed interest of the aggregate variable interest rate borrowings expected to be outstanding under credit facility.

+ References

No definition available.

+ Details

Name:	gnrt_PercentageOfFixedInterestRateOnAggregateVariableInterestRateBorrowings
Namespace Prefix:	gnrt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash flow received from counterparties from interest rate swap modifications.

+ References

No definition available.

+ Details

Name:	gnrt_ProceedsFromCounterpartyFromInterestRateSwapModifications
Namespace Prefix:	gnrt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Gain (loss) on hedging derivative instrument or the foreign currency transaction gain (loss) on the nonderivative hedging instrument designated as a hedge of a net investment in foreign operations related to hedge ineffectiveness, which is included in earnings in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

+ Details

Name:	us-gaap_AmountOfIneffectivenessOnNetInvestmentHedges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

+ Details

Name:	us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Percentage of an entity's outstanding debt that is designated as a hedged item in cash flow, fair value, or net investment hedge.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=gnrt_OtherExpenseIncomeNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

FINANCIAL INSTRUMENTS - Interest Rate Risk Management (Details) $ in Thousands		3 Months Ended		6 Months Ended	12 Months Ended
	Apr. 10, 2017 item	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Dec. 31, 2016 item	May 02, 2016 item
Interest Rate Swap Agreements
Number of interest rate swap transactions \| item						6
Interest rate swap
Interest Rate Swap Agreements
Payment received from swap counterparties		$ 18,200
Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Amounts in accumulated other comprehensive loss expected to be reclassified into earings in the coming 12 months				$ 4,000
Interest rate swap \| Cash flow hedges \| Minimum
Interest Rate Swap Agreements
Swap offsetting percentage of hedged interest payments		80.00%		80.00%
Interest rate swap \| Cash flow hedges \| Maximum
Interest Rate Swap Agreements
Hedge effectiveness (as a percent)		125.00%		125.00%
Interest rate swap \| Cash flow hedges \| Other income (expense), net
Interest Rate Swap Agreements
Hedge ineffectiveness recognized in earnings		$ 2,800	$ 1,600	$ 2,100
Refinancing Facility
Interest Rate Swap Agreements
Number of interest rate swap transactions \| item		2		2
Percentage of fixed interest rate on aggregate variable interest rate borrowings		35.00%		35.00%
Refinancing Facility \| Interest rate swap
Interest Rate Swap Agreements
Number of swap agreements with payment dates modified \| item	2				2
Number of remaining Interest Rate Swaps With Modified Payment Dates \| item	4
Refinancing Facility One \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 94,814		$ 94,814
Derivative, Fixed Interest Rate		1.66%		1.66%
Refinancing Facility Two \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 23,703		$ 23,703
Derivative, Fixed Interest Rate		1.648%		1.648%
Korean Export Credit Facility
Interest Rate Swap Agreements
Number of interest rate swap transactions \| item		3		3
Percentage of fixed interest rate on aggregate variable interest rate borrowings		74.00%		74.00%
Korean Export Credit Facility \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Number of interest rate swap transactions \| item		3		3
Initial aggregate notional amount		$ 599,700		$ 599,700
Maximum aggregate notional amount		610,000		610,000
Korean Export Credit Facility One \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 474,400		$ 474,400
Derivative, Fixed Interest Rate		1.838%		1.838%
Korean Export Credit Facility Two \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 88,950		$ 88,950
Derivative, Fixed Interest Rate		1.8645%		1.8645%
Korean Export Credit Facility Three \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 29,650		$ 29,650
Derivative, Fixed Interest Rate		1.818%		1.818%
Sinosure Credit Facility
Interest Rate Swap Agreements
Percentage of fixed interest rate on aggregate variable interest rate borrowings		100.00%		100.00%
Sinosure Credit Facility \| Interest rate swap \| Cash flow hedges
Interest Rate Swap Agreements
Initial aggregate notional amount		$ 328,652		$ 328,652
Derivative, Fixed Interest Rate		2.047%		2.047%