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Costs of sales (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Costs of sales    
Cost of stream sales $ 102.9 $ 127.4
Cost of prepaid ounces 7.1 7.7
Mineral production taxes 2.3 2.3
Energy operating costs 5.9 4.6
Total costs of sales $ 118.2 $ 142.0