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Goodwill - Changes in carrying amount and impairment testing (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of changes in goodwill [abstract]
Goodwill at beginning of period	$ 205,287	$ 230,210
Impairment		(30,618)	$ (194,612)
Exchange differences	(2,439)	5,695
Goodwill at end of period	202,848	205,287	230,210
Electrometallurgy - North America
Reconciliation of changes in goodwill [abstract]
Goodwill at beginning of period	205,287
Goodwill at end of period	202,848	205,287
Electrometallurgy United States Segment
Reconciliation of changes in goodwill [abstract]
Goodwill at end of period	172,900
Electrometallurgy Canada Segment
Reconciliation of changes in goodwill [abstract]
Goodwill at end of period	29,900
Globe Specialty Metals Inc
Reconciliation of changes in goodwill [abstract]
Goodwill at beginning of period	205,287	230,210
Impairment		(30,618)
Exchange differences	(2,439)	5,695
Goodwill at end of period	202,848	205,287	$ 230,210
Globe Specialty Metals Inc | Electrometallurgy United States Segment
Reconciliation of changes in goodwill [abstract]
Goodwill at end of period	172,913
Globe Specialty Metals Inc | Electrometallurgy Canada Segment
Reconciliation of changes in goodwill [abstract]
Goodwill at end of period	$ 29,935
CANADA | Electrometallurgy - North America
Reconciliation of changes in goodwill [abstract]
Impairment		$ (30,618)

X

- Definition

The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]]

+ References

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- Definition

The amount of impairment loss recognised in profit or loss for goodwill. [Refer: Impairment loss recognised in profit or loss; Goodwill]

+ References

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- Definition

The increase (decrease) in goodwill resulting from net exchange differences arising on the translation of the financial statements from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Goodwill]

+ References

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