LEASES - Schedule of Supplemental Statements of Financial Position (Details) - CAD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Leases, Operating [Abstract]
Operating lease right-of-use assets, net	$ 785	$ 669
Operating lease liabilities - current	121	98
Operating lease liabilities - long-term	738	652
Total operating lease liabilities	$ 859	$ 750
Operating lease, right-of-use asset, statement of financial position [Extensible List]	Deferred amounts and other assets	Deferred amounts and other assets
Operating lease, liability, current, statement of financial position [Extensible List]	Trade payables and accrued liabilities	Trade payables and accrued liabilities
Operating lease, liability, noncurrent, statement of financial position [Extensible List]	Other long-term liabilities	Other long-term liabilities
Finance leases
Finance lease right-of-use assets, net	$ 294	$ 287
Finance lease liabilities - current	16	19
Finance lease liabilities - long-term	300	264
Total finance lease liabilities	$ 316	$ 283
Finance lease, right-of-use asset, statement of financial position [Extensible List]	Property, plant and equipment, net (Note 11)	Property, plant and equipment, net (Note 11)
Finance lease, liability, current, statement of financial position [Extensible List]	Current portion of long-term debt (Note 17)	Current portion of long-term debt (Note 17)
Finance lease, liability, noncurrent, statement of financial position [Extensible List]	Other long-term liabilities	Other long-term liabilities
Weighted average remaining lease term
Operating leases (in years)	14 years	12 years
Finance leases (in years)	31 years	31 years
Weighted average discount rate
Operating leases (as percent)	4.80%	4.50%
Finance leases (as percent)	5.80%	5.70%
Affiliated Entity
Leases, Operating [Abstract]
Operating lease right-of-use assets, net	$ 42	$ 42
Operating lease liabilities - current	6	5
Operating lease liabilities - long-term	$ 37	$ 38