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STOCK-BASED COMPENSATION - Principal assumptions used to determine fair value of stock options (Details) - CAD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Share-based Payment Arrangements [Abstract]    
Weighted average fair value $ 5.86 $ 8.11
Risk-free interest rate 1.70% 1.90%
Dividend yield 3.40% 2.80%
Volatility of Class B Non-Voting Shares 16.10% 16.40%
Weighted average expected life 5.5 5.5