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STOCK BASED COMPENSATION - Stock options exercisable (Details) - Stock options
$ in Millions
12 Months Ended
Mar. 23, 2018
Options
shares
Dec. 31, 2017
CAD ($)
Options
Y
shares
Dec. 31, 2017
USD ($)
Options
Y
shares
Dec. 31, 2016
USD ($)
Options
Y
shares
Dec. 31, 2016
CAD ($)
Options
shares
Dec. 31, 2015
CAD ($)
Options
Share based compensation            
Number of stock options granted | Options     2,018,140 2,160,075    
Number of stock options vested within 30 days of grant date | shares     499,796 540,027    
Vesting period     3 years      
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Number of stock options outstanding | Options   5,857,504     5,478,837 12,082,212
Weighted average remaining contractual life of excercisable stock options   2 years 2 months 1 day        
Weighted average exercise price of outstanding stock options   $ 41.18     $ 34.40 $ 43.65
Number of exercisable stock options | Options   2,628,998     1,606,558  
Weighted average exercise price of exercisable stock options   $ 37.66     $ 40.27  
Common stock reserved for future issuance | shares   5,857,504        
Number of common shares available for grant | shares   4,371,663     6,289,059  
Significant assumptions used to estimate the fair value of stock options            
Risk-free interest rate     1.15% 0.89%    
Expected life of stock options (in years) | Y     2.3 2.5    
Expected volatility (as a percent)     45.00% 45.00%    
Expected dividend yield     1.09% 1.33%    
Total compensation expense recorded in the general and administrative     $ 19.5 $ 16.6    
Share based compensation cost capitalized as part of the property, plant and mine development     $ 0.3 $ 0.3    
Subsequent events            
Share based compensation            
Number of stock options granted | Options 1,990,850          
Number of stock options vested within 30 days of grant date | shares 496,973          
Vesting period 3 years          
Exercise price range 28.03 to 38.15            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Number of stock options outstanding | Options   3,638,052        
Weighted average remaining contractual life of outstanding stock options | Y   2.30        
Weighted average exercise price of outstanding stock options   $ 32.10        
Number of exercisable stock options | Options   1,903,646        
Weighted average exercise price of exercisable stock options   $ 31.05        
Exercise price range 40.66 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Number of stock options outstanding | Options   2,219,452        
Weighted average remaining contractual life of outstanding stock options | Y   3.52        
Weighted average exercise price of outstanding stock options   $ 56.05        
Number of exercisable stock options | Options   725,352        
Weighted average exercise price of exercisable stock options   $ 55.01        
Exercise price range 28.03 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Number of stock options outstanding | Options   5,857,504        
Weighted average remaining contractual life of outstanding stock options | Y   2.76        
Weighted average exercise price of outstanding stock options   $ 41.18        
Number of exercisable stock options | Options   2,628,998        
Weighted average exercise price of exercisable stock options   $ 37.66        
Minimum | Exercise price range 28.03 to 38.15            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   28.03        
Minimum | Exercise price range 40.66 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   40.66        
Minimum | Exercise price range 28.03 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   28.03        
Maximum | Exercise price range 28.03 to 38.15            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   38.15        
Maximum | Exercise price range 40.66 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   66.57        
Maximum | Exercise price range 28.03 to 66.57            
Range of exercise prices and number and weighted average remaining contractual life of stock options            
Range of exercise prices   $ 66.57