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Fair value measurements (Details)
 3 Months Ended
Mar. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Disclosure of fair value measurement of assets [line items]    
Transfer from Level 1 to 2 of financial assets $ 0  
Transfer from Level 2 to 1 of financial assets 0  
Transfer from Level 1 to 2 of financial liabilities 0  
Transfer from Level 2 to 1 of financial liabilities 0  
Transfers Into Level 3 of financial assets 0  
Transfers Out of Level 3 of financial assets 0  
Transfers Into Level 3 of financial liabilities 0  
Transfers Out of Level 3 of financial liabilities 0  
Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 146,700,000 $ 691,300,000
Recurring Basis | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 134,000,000 679,200,000
Recurring Basis | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 8,700,000 12,100,000
Recurring Basis | Level 3    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 4,000,000  
Cash and cash equivalents | Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value   511,100,000
Cash and cash equivalents | Recurring Basis | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value   511,100,000
Receivables from provisional gold equivalents sales | Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 7,900,000 11,300,000
Receivables from provisional gold equivalents sales | Recurring Basis | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value $ 7,900,000 11,300,000
Equity investments    
Disclosure of fair value measurement of assets [line items]    
Number of investments with no quoted market price in an active market | item 1  
Equity investments | Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value $ 138,000,000  
Equity investments | Recurring Basis | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 134,000,000  
Equity investments | Recurring Basis | Level 3    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 4,000,000  
Available-for-sale | Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value   168,100,000
Available-for-sale | Recurring Basis | Level 1    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value   168,100,000
Warrants | Recurring Basis    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value 800,000 800,000
Warrants | Recurring Basis | Level 2    
Disclosure of fair value measurement of assets [line items]    
Financial assets, at fair value $ 800,000 $ 800,000