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COMMON SHARES - Stock Options Assumptions Used (Details) - CAD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]      
Weighted average fair value (in dollars per share) $ 6.37 $ 5.80 $ 7.22
Expected life 5 years 8 months 12 days 5 years 8 months 12 days 5 years 8 months 12 days
Interest rate 1.90% 2.10% 1.20%
Volatility 19.00% 16.00% 18.00%
Dividend yield 5.00% 4.20% 3.60%
Expense for stock options $ 13 $ 13 $ 12
Unrecognized compensation costs related to non-vested stock options $ 14    
Employee Stock Option      
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]      
Expense recognition period 1 year 8 months 12 days