Commitments and Contingencies (Details)	3 Months Ended
Sep. 30, 2012
Employee Contracts [Abstract]
Number of employees covered under union agreement	322
New York Power Authority [Member] \| Niagara Falls, New York [Member]
Commitment [Line Items]
Capacity	40,000 kW

Energy volume that must be purchased within the specified time period of the agreement to avoid penalties or unfavorable effects on pricing, among other adverse consequences.

No definition available.

Name:	gsm_Long-TermPurchaseCommitmentMinimumEnergyVolumeRequiredDescription
Namespace Prefix:	gsm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration