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EQUITY - Schedule Using Black-Scholes Module, DSU And RSU (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
year
Dec. 31, 2016
USD ($)
year
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Management Share Option Plan | Equity-Settled Share-Based Payment Arrangement        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Weighted-average share price (in dollars per share) $ 36.92 $ 30.59    
Weighted-average fair value per option (in dollars per share) $ 4.92 $ 5.29    
Average term to exercise | year 7.5 7.5    
Share price volatility 18.90% 28.00%    
Liquidity discount 25.00% 25.00%    
Weighted-average annual dividend yield 2.10% 1.60%    
Risk-free rate 2.30% 1.60%    
Escrowed Stock Plan | Equity-Settled Share-Based Payment Arrangement        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Weighted-average share price (in dollars per share) $ 36.88 $ 30.59    
Weighted-average fair value per option (in dollars per share) $ 4.92 $ 5.29    
Average term to exercise | year 7.5 7.5    
Share price volatility 18.90% 28.00%    
Liquidity discount 25.00% 25.00%    
Weighted-average annual dividend yield 2.10% 1.60%    
Risk-free rate 2.30% 1.60%    
Deferred Stock Unit | Cash-Settled Share-Based Payment Arrangement        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Share price on date of measurement (in cad per share)     $ 54.72 $ 44.30
Share price on date of measurement (in usd per share) $ 43.54 $ 33.01    
Restricted Stock | Cash-Settled Share-Based Payment Arrangement        
Disclosure of terms and conditions of share-based payment arrangement [line items]        
Share price on date of measurement (in cad per share)     54.72 44.30
Weighted-average fair value per option (in dollars per share)     $ 45.63 $ 35.21