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PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about property, plant and equipment [line items]      
Property, plant and equipment $ 12,167 $ 12,198 $ 11,780
Property, plant and equipment, weighted average capitalised interest rate 3.70% 3.90%  
Disposals, property, plant and equipment   $ 38  
Proceeds on disposition $ 0 38  
Adjustment to cost and accumulated depreciation 978 1,159  
Construction in progress (not yet in service)      
Disclosure of detailed information about property, plant and equipment [line items]      
Property, plant and equipment $ 848 $ 1,320