SEGMENTED INFORMATION - GEOGRAPHIC INFORMATION (Details) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Geographic Information
Revenues	$ 10,008	$ 9,110	$ 7,956	$ 12,013	$ 12,352	$ 11,598	$ 13,263	$ 12,856	$ 39,087	$ 50,069	$ 46,378
Property, plant and equipment, net	94,571				93,723				94,571	93,723	94,540
Canada
Geographic Information
Revenues									16,453	19,954	19,023
Property, plant and equipment, net	46,499				45,993				46,499	45,993
US
Geographic Information
Revenues									22,634	30,115	$ 27,355
Property, plant and equipment, net	$ 48,072				$ 47,730				$ 48,072	$ 47,730