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Other liabilities - Contingent Consideration (Details) - USD ($)
$ in Thousands
12 Months Ended
Feb. 01, 2018
Dec. 31, 2019
Dec. 31, 2018
Kintuk      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Percentage of equity interests acquired 100.00%    
Contingent consideration $ 26,222 $ 21,965 $ 26,222
Ferroglobe Mangan Norge and Ferroglobe Manganese France      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Period for contingent consideration 8 years 6 months    
Ferroglobe Mangan Norge and Ferroglobe Manganese France | Minimum      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Contingent consideration arrangements $ 0    
Ferroglobe Mangan Norge and Ferroglobe Manganese France | Maximum      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Contingent consideration arrangements $ 60,000    
Ferroglobe Mangan Norge      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Discount rates   11.50% 11.50%
Average simulated revenues   $ 157,276 $ 269,256
Ferroglobe Manganese France      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Discount rates   11.00% 11.00%
Average simulated revenues   $ 317,507 $ 312,526