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Equity - Share capital (Details) € / shares in Units, $ / shares in Units, € in Thousands |  | 12 Months Ended |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 13, 2015 EUR (€) | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) Option $ / shares shares | Dec. 31, 2016 USD ($) | Jun. 22, 2016 $ / shares | Jun. 21, 2016 $ / shares | Dec. 31, 2015 USD ($) | Dec. 23, 2015 € / shares shares | Dec. 23, 2015 $ / shares shares | Oct. 14, 2015 $ / shares | Oct. 13, 2015 USD ($) $ / shares shares | Feb. 05, 2015 $ / shares shares
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in shares) |  | 40,000 | 139,000 |  |  |  |  |  |  |  |  |
Issue of equity | € 50,000 | $ 240,000 | $ 180,000 |  |  |  |  |  |  |  |  |
Par value per share | $ / shares |  |  | $ 0.01 |  | $ 0.01 | $ 7.50 |  |  |  |  |  |
Number of shares outstanding |  | 169,122,682 |  |  |  |  |  |  |  |  |  |
Number of shares outstanding, including treasury shares |  | 170,863,773 | 171,976,731 |  |  |  |  |  |  |  |  |
Treasury shares | $ |  | $ 1,174,091 |  |  |  |  |  |  |  |  |  |
Number of ordinary shares issued |  | 40,000 | 138,578 |  |  |  |  |  |  |  |  |
Number of shares issued at vesting of restricted stock units |  |  | 108,578 |  |  |  |  |  |  |  |  |
Number of share options exercised in share-based payment arrangement | Option |  |  | 30,000 |  |  |  |  |  |  |  |  |
Equity | $ |  | $ 884,372,000 | $ 937,758,000 | $ 892,042,000 |  |  | $ 1,294,973,000 |  |  |  |  |
Globe Specialty Metals Inc |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Par value per share | $ / shares |  |  |  |  |  |  |  |  | $ 0.0001 |  |  |
Ordinary shares [member] |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in shares) |  |  |  |  |  |  |  | 1 | 1 |  | 14 | 1
Par value per share | $ / shares |  |  |  |  |  |  |  |  | $ 7.50 | $ 7.50 | $ 1.00 | $ 1.00
Shares consolidated |  |  |  |  |  |  |  |  |  |  | 15 |
New shares issued after consolidation |  |  |  |  |  |  |  |  |  |  | 2 |
Equity | $ |  |  |  |  |  |  |  |  |  |  | $ 15.00 |
Class A |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in shares) |  |  |  |  |  |  |  | 98,078,161 | 98,078,161 |  |  |
Par value per share | $ / shares |  | $ 0.01 |  |  |  |  |  |  | $ 7.50 |  |  |
Preference shares [member] |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Shares issued (in shares) |  |  |  |  |  |  |  |  |  |  | 50,000 |
Grupo Villar Mir SAU [Member] |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
The number of beneficially shares owned |  | 91,125,521 |  |  |  |  |  |  |  |  |  |
Outstanding shares owned by main shareholders (as a percentage) |  | 53.90% |  |  |  |  |  |  |  |  |  |
Grupo Villar Mir SAU [Member] | Grupo FerroAtlantica S.A.U. [Member] |  |  |  |  |  |  |  |  |  |  |  |
Disclosure of classes of share capital [line items] |  |  |  |  |  |  |  |  |  |  |  |
Par value per share | € / shares |  |  |  |  |  |  |  | € 1,000 |  |  |  |

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