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Mining interests - Carrying amount (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Mining Interests    
Property, plant and equipment $ 1,787.9 $ 1,828.3
New Afton    
Mining Interests    
Property, plant and equipment 818.6 687.8
Rainy River    
Mining Interests    
Property, plant and equipment 966.7 987.2
Other    
Mining Interests    
Property, plant and equipment 2.6 153.3
Depletable    
Mining Interests    
Property, plant and equipment 740.2 660.3
Depletable | New Afton    
Mining Interests    
Property, plant and equipment 396.5 382.9
Depletable | Rainy River    
Mining Interests    
Property, plant and equipment 343.7 277.4
Depletable | Other    
Mining Interests    
Property, plant and equipment 0.0 0.0
Non- depletable    
Mining Interests    
Property, plant and equipment 261.0 300.4
Non- depletable | New Afton    
Mining Interests    
Property, plant and equipment 224.0 127.5
Non- depletable | Rainy River    
Mining Interests    
Property, plant and equipment 35.9 21.8
Non- depletable | Other    
Mining Interests    
Property, plant and equipment 1.1 151.1
Plant & equipment    
Mining Interests    
Property, plant and equipment 668.6 710.6
Plant & equipment | New Afton    
Mining Interests    
Property, plant and equipment 154.0 150.0
Plant & equipment | Rainy River    
Mining Interests    
Property, plant and equipment 513.1 558.4
Plant & equipment | Other    
Mining Interests    
Property, plant and equipment 1.5 2.2
Construction in progress    
Mining Interests    
Property, plant and equipment 118.1 157.0
Construction in progress | New Afton    
Mining Interests    
Property, plant and equipment 44.1 27.4
Construction in progress | Rainy River    
Mining Interests    
Property, plant and equipment 74.0 129.6
Construction in progress | Other    
Mining Interests    
Property, plant and equipment $ 0.0 $ 0.0