RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Level 3 Inputs (Details) $ in Millions	Sep. 30, 2024 CAD ($) $ / MWh $ / bbl $ / MillionsofBTU-MMBTU	Dec. 31, 2023 CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (1,161)	$ (404)
Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(1,161)	(404)
Level 3
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(109)
Level 3 | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(109)	$ (131)
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (5)
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	2.65
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	8.64
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	4.47
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (10)
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	68.33
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	93.84
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	89.52
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (75)
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	28.63
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	162.60
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	60.57
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (10)
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	0.05
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	10.03
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MillionsofBTU-MMBTU	3.66
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ 25
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	70.48
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	111.10
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / bbl	90.27
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (73)
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	22.77
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	144.39
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent) | $ / MWh	61.52
Market approach valuation technique | Level 3 | Commodity Options | Natural gas | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ 39
Market approach valuation technique | Level 3 | Commodity Options | Natural gas | Minimum Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent)	0.06
Market approach valuation technique | Level 3 | Commodity Options | Natural gas | Maximum Price/ Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent)	0.70
Market approach valuation technique | Level 3 | Commodity Options | Natural gas | Weighted Average Price/Volatility
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H, percent)	0.46