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Fair value measurements (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2022
USD ($)
EquityInstruments
Jun. 30, 2022
USD ($)
EquityInstruments
Dec. 31, 2021
USD ($)
Fair value measurements      
Transfer from Level 1 to 2 of financial assets $ 0 $ 0  
Transfer from Level 2 to 1 of financial assets 0 0  
Transfer from Level 1 to 2 of financial liabilities 0 0  
Transfer from Level 2 to 1 of financial liabilities 0 0  
Transfers Into Level 3 of financial assets 0 0  
Transfers Out of Level 3 of financial assets 0 0  
Transfers Into Level 3 of financial liabilities 0 0  
Transfers Out of Level 3 of financial liabilities $ 0 $ 0  
Number of investments with no quoted market price in an active market | EquityInstruments 2 2  
Recurring Basis      
Fair value measurements      
Financial assets, at fair value $ 220,600,000 $ 220,600,000 $ 240,700,000
Recurring Basis | Level 1      
Fair value measurements      
Financial assets, at fair value 169,200,000 169,200,000 231,000,000.0
Recurring Basis | Level 2      
Fair value measurements      
Financial assets, at fair value 10,000,000.0 10,000,000.0 5,600,000
Recurring Basis | Level 3      
Fair value measurements      
Financial assets, at fair value 41,400,000 41,400,000 4,100,000
Receivables from provisional concentrate sales | Recurring Basis      
Fair value measurements      
Financial assets, at fair value 7,400,000 7,400,000 4,800,000
Receivables from provisional concentrate sales | Recurring Basis | Level 2      
Fair value measurements      
Financial assets, at fair value 7,400,000 7,400,000 4,800,000
Equity investments | Recurring Basis      
Fair value measurements      
Financial assets, at fair value 210,600,000 210,600,000 235,100,000
Equity investments | Recurring Basis | Level 1      
Fair value measurements      
Financial assets, at fair value 169,200,000 169,200,000 231,000,000.0
Equity investments | Recurring Basis | Level 3      
Fair value measurements      
Financial assets, at fair value 41,400,000 41,400,000 4,100,000
Warrants. | Recurring Basis      
Fair value measurements      
Financial assets, at fair value 2,600,000 2,600,000 800,000
Warrants. | Recurring Basis | Level 2      
Fair value measurements      
Financial assets, at fair value $ 2,600,000 $ 2,600,000 $ 800,000