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Provisions - Other Benefit Plans (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) plan	Dec. 31, 2017 USD ($)
Other provisions [abstract]
Provisions for litigation	$ 4,450	$ 8,900
Litigation paid	$ 4,450
Globe Metallurgical Inc [Member]
Other Benefit Plans [Abstract]
Company match percentage	25.00
Globe Metallurgical Inc [Member] | North America
Other Benefit Plans [Abstract]
Number of defined contribution plans provided | plan	2
Maximum | Globe Metallurgical Inc [Member]
Other Benefit Plans [Abstract]
Percentage of compensation	6.00
Asbestos-related claims
Other provisions [abstract]
Bond to guarantee civil financial responsibility	$ 1,775

X

- Definition

The amount of bond to guarantee civil financial responsibility

+ References

No definition available.

+ Details

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

X

- Definition

Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

+ Details

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Namespace Prefix:	gsm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	gsm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the number of defined contribution plans provided.

+ References

No definition available.

+ Details

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Namespace Prefix:	gsm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

n/a

+ References

No definition available.

+ Details

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X

- Definition

The cash outflows for payment of litigation.

+ References

No definition available.

+ Details

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Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of provisions related to litigation.

+ References

No definition available.

+ Details

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Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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X

- Details

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X

- Details

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Balance Type:
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X

- Details

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X

- Details

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