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Share Capital (Flow-though share financing) (Details)
$ / shares in Units, CAD in Millions, $ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2017
$ / shares
Dec. 31, 2017
CAD
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Flow-though Share Financing [Line Items]        
Proceeds from issuance of flow-through shares | $     $ 11.7 $ 20.4
Canadian Development Expense [Member]        
Flow-though Share Financing [Line Items]        
Proceeds from issuance of flow-through shares | CAD   CAD 12.0    
Proceeds from flow-through share financing (CAD per share) | $ / shares $ 10.87      
Canadian Exploration Expense [Member]        
Flow-though Share Financing [Line Items]        
Proceeds from issuance of flow-through shares | CAD   CAD 3.0    
Proceeds from flow-through share financing (CAD per share) | $ / shares $ 15.15