RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Level 3 Inputs (Details) $ in Millions	Jun. 30, 2021 CAD ($) $ / bbl $ / MWh $ / Gallon-gal $ / MillionsofBTU-MMBTU	Dec. 31, 2020 CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (222)	$ (1,069)
Recurring basis
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(222)	(1,069)
Level 3 | Recurring basis
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (178)	$ (191)
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	2.83
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	5.90
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Natural gas | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	3.87
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	67.09
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	90.65
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Crude | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	75.20
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	0.99
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	1.39
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | NGL | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	1.37
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	18.20
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	116.93
Market approach valuation technique | Level 3 | Commodity Contracts - Financial | Power | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	63.92
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	2.43
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	6.75
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Natural gas | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	4.02
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	67.19
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	91.39
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | Crude | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	85.53
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Minimum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	0
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Maximum
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	0
Market approach valuation technique | Level 3 | Commodity Contracts - Physical | NGL | Weighted Average
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / Gallon-gal	0
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Natural gas
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (9)
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Crude
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(17)
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | NGL
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	0
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Financial | Power
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(49)
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | Natural gas
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(17)
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | Crude
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(86)
Market approach valuation technique | Level 3 | Recurring basis | Commodity Contracts - Physical | NGL
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ 0