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Shareholders' equity - Stock-based payments - Assumptions & Options (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
$ / shares
Sep. 30, 2018
USD ($)
EquityInstruments
Y
Sep. 30, 2017
USD ($)
EquityInstruments
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Stock-based payments          
Capitalized to royalty, stream and working interest, net   $ 0 $ 0 $ 300,000 $ 200,000
Stock options          
Stock-based payments          
Term of options   10 years      
Vesting period   3 years      
Weighted average fair value per stock option (in dollars per share) | $ / shares $ 25.14        
Risk-free interest rate   2.18%      
Expected dividend yield   1.41%      
Expected price volatility of the Company's common shares   33.30%      
Expected life of the option | Y   5      
Forfeiture rate   0.00%      
Expenses related to stock options   $ 1,500,000 $ 900,000 $ 4,100,000 $ 4,000,000
Granted (in shares) | EquityInstruments   47,732 0    
Granted (in dollars per share)   $ 88.76      
Fair value of options granted   $ 900,000