Provisions - Employee obligations - Venezuela - (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Venezuela
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase	7374.00%	400000.00%
Discount rate	7673.00%	520004.00%
Expected inflation rate	7374.00%	500000.00%
Actuarial assumption of retirement age		64 years
Venezuela \| Minimum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Actuarial assumption of retirement age	62 years
Venezuela \| Maximum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Actuarial assumption of retirement age	63 years
France
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Discount rate	0.75%	2.00%
Expected inflation rate	1.60%	1.60%
Actuarial assumption of retirement age	65 years	65 years
France \| Minimum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase	1.60%	1.60%
France \| Maximum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase	6.10%	6.10%
South Africa
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase		7.20%
Discount rate		9.90%
Expected inflation rate		6.20%
Actuarial assumption of retirement age	63 years	63 years
South Africa \| Minimum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase	7.10%
Discount rate	9.50%
Expected inflation rate	5.10%
South Africa \| Maximum
Actuarial Assumptions Used To Calculate Defined Benefit Obligation [Abstract]
Salary increase	7.60%
Discount rate	10.70%
Expected inflation rate	6.10%
FerroAtlantica De Venezuela SA \| Venezuela
Disclosure of defined benefit plans [line items]
Years of service	15 years
Percentage of basic salary guaranteed by IVSS	80
Present value of defined benefit obligation [member] \| South Africa
Changes in net defined benefit liability (asset) [abstract]
Obligations at beginning of year	$ 5,429	$ 7,872
Current service cost	90	139
Borrowing costs	511	740
Actuarial differences	1,291	2,000
Benefits paid	254	226
Exchange differences	116	(1,096)
Obligations at end of year	4,601	5,429
Present value of defined benefit obligation [member] \| FerroAtlantica De Venezuela SA \| Venezuela
Changes in net defined benefit liability (asset) [abstract]
Obligations at beginning of year	534	1,883
Current service cost	50	775
Borrowing costs	1,128
Benefits paid	(3)	(35)
Exchange differences	(1,200)	(2,089)
Other	2,068
Obligations at end of year	$ 2,577	$ 534

n/a

No definition available.

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Represents the number of years of service at the company accumulated by the employee

+ References

No definition available.

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Represents the percentage of basic salary guaranteed by Venezuelan Social Security Institute (IVSS).

+ References

No definition available.

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- Definition

The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Number 19
-IssueDate 2019-01-01
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- Definition

The expected rate of inflation that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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- Definition

The expected rate of salary increases used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]]

+ References

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The retirement age used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]]

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The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

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The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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