INTANGIBLE ASSETS (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INTANGIBLE ASSETS
Cost	$ 3,532	$ 3,522
Accumulated Amortization	(1,359)	(1,150)
Net	2,173	2,372
Amortization expenses
Amortization expense for intangible assets	296	$ 281	$ 280
Expected amortization expense for intangible assets
2020	292
2021	263
2022	238
2023	216
2024	$ 195
Customer relationships
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	5.40%	5.70%
Cost	$ 861	$ 889
Accumulated Amortization	(231)	(187)
Net	$ 630	$ 702
Power purchase agreements
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	4.50%	5.40%
Cost	$ 64	$ 82
Accumulated Amortization	(16)	(15)
Net	$ 48	$ 67
Project agreement
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	4.00%	4.00%
Cost	$ 156	$ 164
Accumulated Amortization	(16)	(10)
Net	$ 140	$ 154
Software
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	11.20%	10.00%
Cost	$ 1,988	$ 1,902
Accumulated Amortization	(1,014)	(875)
Net	$ 974	$ 1,027
Other intangible assets
INTANGIBLE ASSETS
Weighted Average Amortization Rate (as a percent)	2.90%	2.00%
Cost	$ 463	$ 485
Accumulated Amortization	(82)	(63)
Net	$ 381	$ 422