RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Other Financial Instruments (Details) - CAD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Fair Value of Other Financial Instruments
Transfer into or out of level 3			$ 0		$ 0
FMVA investments	$ 185,000,000		185,000,000		187,000,000
Long-term debt	98,700,000,000		98,700,000,000		98,900,000,000
Equity Funds And Debt Securities
Fair Value of Other Financial Instruments
Unrealized gain on investment	1,000,000	$ 5,000,000	1,000,000	$ 21,000,000
Level 1 | Equity Funds
Fair Value of Other Financial Instruments
Fair value of investments	0		0		114,000,000
Level 2
Fair Value of Other Financial Instruments
Restricted long-term investments, cost basis	1,100,000,000		1,100,000,000		1,100,000,000
Level 2 | Debt Securities
Fair Value of Other Financial Instruments
Fair value of investments	1,100,000,000		1,100,000,000		1,100,000,000
Carrying Value
Fair Value of Other Financial Instruments
Long-term debt	$ 100,600,000,000		100,600,000,000		$ 101,600,000,000
Net investment hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange (loss) gain on translation of United States dollar denominated debt			494,000,000	(425,000,000)
Realized loss associated with the settlement of foreign exchange forward contracts			$ 81,000,000	$ 113,000,000