RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Other Financial Instruments (Details) - CAD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value of Other Financial Instruments
Unrealized gains (losses) on the change in fair value of foreign exchange foreign contracts			$ 0	$ 0
Realized gain (loss) recognized in OCI associated with settlement of foreign exchange forward contracts			0	0
FMVA investments	$ 209,000,000		209,000,000		$ 102,000,000
Restricted long-term investments	635,000,000		635,000,000		593,000,000
Long-term debt, carrying value	76,300,000,000		76,300,000,000		79,300,000,000
Long-term debt	68,900,000,000		68,900,000,000		73,500,000,000
Debt Securities
Fair Value of Other Financial Instruments
Debt securities, available-for-sale, amortized cost	346,000,000		346,000,000		295,000,000
Equity Funds And Debt Securities
Fair Value of Other Financial Instruments
Unrealized gain (loss) on lnvestments	(8,000,000)	$ (13,000,000)	14,000,000	(40,000,000)
Net investment hedges
Fair Value of Other Financial Instruments
Unrealized foreign exchange gain (loss) on translation of United States dollar denominated debt			86,000,000	(1,191,000,000)
Realized loss associated with the settlement of foreign exchange forward contracts			44,000,000	0
Level 1
Fair Value of Other Financial Instruments
Restricted long-term investments	252,000,000		252,000,000		236,000,000
Level 1 | Debt Securities
Fair Value of Other Financial Instruments
Fair value of investments	273,000,000		273,000,000		335,000,000
Level 2
Fair Value of Other Financial Instruments
Restricted long-term investments	383,000,000		383,000,000		357,000,000
Restricted long-term investments, cost basis	474,000,000		474,000,000		437,000,000
Level 2 | Debt Securities
Fair Value of Other Financial Instruments
Fair value of investments	353,000,000		353,000,000		298,000,000
Fair Value
Fair Value of Other Financial Instruments
Restricted long-term investments	635,000,000		635,000,000		593,000,000
Unrealized holding gains (losses)	(44,000,000)	$ 11,000,000	(7,000,000)	$ (120,000,000)
Fair value note receivable	660,000,000		660,000,000		752,000,000
Carrying value
Fair Value of Other Financial Instruments
Non-current notes receivable	$ 660,000,000		$ 660,000,000		$ 752,000,000