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Shareholders' equity - Stock-based payments - Assumptions & Options (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock-based payments    
Capitalized to royalty, stream and working interest, net $ 0.3 $ 0.1
Stock options    
Stock-based payments    
Expenses related to stock options $ 1.2 $ 1.6