INTANGIBLE ASSETS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INTANGIBLE ASSETS
Cost	$ 3,688	$ 3,532
Accumulated Amortization	(1,608)	(1,359)
Net	2,080	2,173
Amortization expenses
Amortization expense for intangible assets	294	$ 296	$ 281
Expected amortization expense for intangible assets
2021	298
2022	270
2023	245
2024	222
2025	$ 202
Customer relationships
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	5.00%	5.00%
Cost	$ 724	$ 734
Accumulated Amortization	(139)	(104)
Net	$ 585	$ 630
Power purchase agreements
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	4.50%	4.50%
Cost	$ 63	$ 64
Accumulated Amortization	(18)	(16)
Net	$ 45	$ 48
Project agreement
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	4.00%	4.00%
Cost	$ 153	$ 156
Accumulated Amortization	(21)	(16)
Net	$ 132	$ 140
Software
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	10.50%	11.00%
Cost	$ 2,292	$ 2,115
Accumulated Amortization	(1,334)	(1,141)
Net	$ 958	$ 974
Other intangible assets
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	2.70%	2.90%
Cost	$ 456	$ 463
Accumulated Amortization	(96)	(82)
Net	$ 360	$ 381