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Share Capital (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Number of issued shares [Line Items]    
Beginning of year 395,792,522 395,792,522
Stock option plan 210 0
End of year 395,792,732 395,792,522
Common shares [member]    
Number of issued shares [Line Items]    
Common shares par value no  
Dividends Paid Ordinary Shares Per Share $ 0.4 $ 0.4
Class B Shares [Member]    
Number of issued shares [Line Items]    
Class B share par value $ 1  
Class B share preferences entitles the shareholder to vote separately as a class in respect of any proposal to locate the head office of Cameco to a place not in the province of Saskatchewan