Aggregate amount of equipment orders.

No definition available.

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Namespace Prefix:	gmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregate amount spend for Mt. Hope project inception-to-date.

+ References

No definition available.

+ Details

Name:	gmo_AggregateAmountSpendForProjectCost
Namespace Prefix:	gmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cumulative amount of costs incurred in the exploration and development of the project, including capitalized costs and costs charged to expense.

+ References

No definition available.

+ Details

Name:	gmo_DevelopmentCost
Namespace Prefix:	gmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Nonrefundable down payment made on a purchase order.

+ References

No definition available.

+ Details

Name:	gmo_NonrefundableDownPayment
Namespace Prefix:	gmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of ball mills.

+ References

No definition available.

+ Details

Name:	gmo_NumberOfBallMills
Namespace Prefix:	gmo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of 230kV primary transformers

+ References

No definition available.

+ Details

Name:	gmo_NumberOfPrimaryTransformers
Namespace Prefix:	gmo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number pieces of equipment for which purchase orders have been issued during the period.

+ References

No definition available.

+ Details

Name:	gmo_NumberOrdered
Namespace Prefix:	gmo_
Data Type:	xbrli:integerItemType
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_OtherCommitmentsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

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

- Details

Name:	dei_LegalEntityAxis=gmo_EurekaMolyLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis=us-gaap_PurchaseCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=gmo_HaulTrucksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=gmo_MineProductionDrillsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=gmo_ElectricShovelsMember
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COMMITMENTS AND CONTINGENCIES - Equipment and Supply Procurement (Details) $ in Millions			12 Months Ended
	Jul. 11, 2012 USD ($)	Jun. 30, 2012 item	Dec. 31, 2012 USD ($) item	Mar. 31, 2017 USD ($) instrument
Equipment and Supply Procurement
Amount committed to be purchased				$ 2.2
Number of primary transformers \| instrument				2
Number of ball mills \| instrument				2
Eureka Moly LLC \| Mt. Hope Project
Equipment and Supply Procurement
Aggregate amount of equipment orders				$ 87.4
Development costs				199.0
Total amount spend for Mt. Hope project inception-to-date				$ 286.4
Purchase Commitment \| Eureka Moly LLC \| Haul trucks
Equipment and Supply Procurement
Nonrefundable down payment			$ 1.2
Number ordered \| item			18
Purchase Commitment \| Eureka Moly LLC \| Mine production drills
Equipment and Supply Procurement
Nonrefundable down payment			$ 0.4
Number ordered \| item			4
Purchase Commitment \| Eureka Moly LLC \| Electric shovels
Equipment and Supply Procurement
Nonrefundable down payment	$ 3.4
Number ordered \| item		2