RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Level 3 Inputs (Details) $ in Millions	Jun. 30, 2024 CAD ($) $ / MillionsofBTU-MMBTU $ / MWh $ / bbl	Dec. 31, 2023 CAD ($)
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	$ (1,326)	$ (404)
Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(1,326)	(404)
Level 3
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(149)
Level 3 | Fair Value
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Fair Value	(149)	$ (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
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	2.51
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	9.48
Market approach valuation technique | Level 3 | Commodity contracts - financial | Natural gas | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	3.43
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	$ 9
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Minimum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	81.65
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	111.75
Market approach valuation technique | Level 3 | Commodity contracts - financial | Crude | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	102.76
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	$ (73)
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Minimum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	26.34
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	166.75
Market approach valuation technique | Level 3 | Commodity contracts - financial | Power | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	62.98
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	$ 13
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Minimum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	0.28
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	10.14
Market approach valuation technique | Level 3 | Commodity contracts - physical | Natural gas | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MillionsofBTU-MMBTU	3.43
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	$ 6
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Minimum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	80.73
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	114.94
Market approach valuation technique | Level 3 | Commodity contracts - physical | Crude | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / bbl	104.81
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	$ (99)
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Minimum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	18.86
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Maximum Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	194.97
Market approach valuation technique | Level 3 | Commodity contracts - physical | Power | Weighted Average Price
Significant unobservable inputs used in the fair value measurement of Level 3 derivative instruments
Forward price ($ / mmbtu, barrel, gallon, MW/H) | $ / MWh	66.91