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STOCK BASED COMPENSATION - Stock options (Details) - Stock options
 12 Months Ended
Dec. 31, 2017
CAD ($)
Options
shares
Dec. 31, 2016
CAD ($)
Options
shares
Share based compensation    
Number of common shares that reserved for issuance to any one person under ESOP (in percentage) 5.00%  
Maximum number of common shares reserved for issuance | shares   31,300,000
Number of stock options vested within 30 days of grant date | shares 499,796 540,027
Vesting period 3 years  
Number of stock options    
Outstanding, beginning of year | Options 5,478,837 12,082,212
Granted during the year | Options 2,018,140 2,160,075
Exercised during the year | Options (1,538,729) (6,492,907)
Forfeited during the year | Options (99,644) (141,038)
Expired during the year | Options (1,100) (2,129,505)
Outstanding, end of year | Options 5,857,504 5,478,837
Options exercisable, end of year | Options 2,628,998 1,606,558
Weighted average exercise price    
Outstanding at beginning of the year $ 34.40 $ 43.65
Granted during the year 56.57 36.65
Exercised during the year 37.18 38.48
Forfeited during the year 42.09 38.42
Expired during the year 37.05 76.46
Options exercisable, end of year 41.18 34.40
Options exercisable, end of year 37.66 40.27
Average share price of common shares 59.47 58.52
Fair value at grant date $ 14.51 $ 9.69
Common stock reserved for future issuance | shares 5,857,504  
Number of common shares available for grant | shares 4,371,663 6,289,059
Maximum    
Share based compensation    
Option term (expiration period) 5 years