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STOCK-BASED COMPENSATION - Principal assumptions used to determine fair value of stock options (Details)
12 Months Ended
Dec. 31, 2018
CAD ($)
step
Dec. 31, 2017
CAD ($)
step
Share-based Payment Arrangements [Abstract]    
Weighted average fair value | $ $ 8.42 $ 8.52
Risk-free interest rate 1.70% 0.80%
Dividend yield 3.30% 3.20%
Volatility of Class B Non-Voting Shares 20.10% 21.20%
Weighted average expected life 6.2 5.5
Weighted average time to vest 2 years 6 months 2 years 3 months 18 days
Weighted average time to expiry 10 years 9 years 10 months 24 days
Employee exit rate 4.90% 3.90%
Suboptimal exercise factor 1.4 1.4
Lattice steps | step 50 50