RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Restricted Long-Term Investments (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Long-Lived Assets Held-for-sale [Line Items]
Purchase of restricted long-term investments	$ 103	$ 36
Sale of restricted long-term investments	77	10
Level 1
Long-Lived Assets Held-for-sale [Line Items]
Restricted long-term investments and cash	529		$ 491
Level 2
Long-Lived Assets Held-for-sale [Line Items]
Restricted long-term investments and cash	536		507
Debt securities, available-for-sale, amortized cost	558		540
Fair Value
Long-Lived Assets Held-for-sale [Line Items]
Restricted long-term investments and cash	1,065		$ 998
Unrealized holding gains (losses)	$ 16	$ (13)