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Mineral Property, Pland and Equipment (Net Value Carrying Values) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment $ 2,753.4 $ 1,918.2
Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 769.1 710.2
Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 1,588.3 867.9
Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 75.4 61.1
Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 320.6 279.0
Operating segments [member] | Young-Davidson [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 1,432.0 1,431.0
Operating segments [member] | Young-Davidson [Member] | Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 589.5 580.7
Operating segments [member] | Young-Davidson [Member] | Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 767.1 789.2
Operating segments [member] | Young-Davidson [Member] | Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 75.4 61.1
Operating segments [member] | Young-Davidson [Member] | Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Operating segments [member] | Mulatos [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 212.0 198.5
Operating segments [member] | Mulatos [Member] | Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 112.9 121.6
Operating segments [member] | Mulatos [Member] | Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 99.1 76.9
Operating segments [member] | Mulatos [Member] | Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Operating segments [member] | Mulatos [Member] | Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Operating segments [member] | Island Gold [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 774.9  
Operating segments [member] | Island Gold [Member] | Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 54.8  
Operating segments [member] | Island Gold [Member] | Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 720.1  
Operating segments [member] | Island Gold [Member] | Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0  
Operating segments [member] | Island Gold [Member] | Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0  
Operating segments [member] | El Chanate [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 6.1 4.4
Operating segments [member] | El Chanate [Member] | Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 4.1 2.6
Operating segments [member] | El Chanate [Member] | Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 2.0 1.8
Operating segments [member] | El Chanate [Member] | Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Operating segments [member] | El Chanate [Member] | Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Corporate and other [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 328.4 284.3
Corporate and other [Member] | Plant and Equipment [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 7.8 5.3
Corporate and other [Member] | Mineral Property - Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Corporate and other [Member] | Mineral Property - Non-Depletable [Member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment 0.0 0.0
Corporate and other [Member] | Exploration and evaluation assets [member]    
Disclosure of detailed information about property, plant and equipment [line items]    
Mineral property, plant and equipment $ 320.6 $ 279.0