RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Level 3 Inputs (Details) $ in Millions	Sep. 30, 2021 CAD ($) $ / bbl $ / Gallon-gal $ / MillionsofBTU-MMBTU $ / MWh	Dec. 31, 2020 CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (576)	$ (1,069)
Recurring basis
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(576)	(1,069)
Level 3 | Recurring basis
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (234)	$ (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	3.34
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	9.66
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	4.95
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	68.14
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	94.91
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	81.51
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
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
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
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	37.91
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	128.70
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	76.20
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	2.86
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	9.85
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	6.30
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	75.66
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	96.75
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	90.70
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	$ (20)
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	(2)
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	(65)
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	(94)
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	(53)
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