PROPERTY, PLANT AND EQUIPMENT - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 3,400	$ 3,000	$ 2,900
Impairment loss		423	$ 1,104
Access Northeast Project
Property, Plant and Equipment [Line Items]
Impairment loss		$ 105