LEASES - Supplemental Statements of Financial Position (Details) - CAD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Leases, Operating [Abstract]
Operating lease right-of-use assets, net	$ 669	$ 680
Operating lease liabilities - current	98	87
Operating lease liabilities - long-term	652	677
Total operating lease liabilities	$ 750	$ 764
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]	Accounts Payable and Other Accrued Liabilities, Current	Accounts Payable and Other Accrued Liabilities, Current
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	$ 287	$ 62
Finance lease liabilities - current	19	17
Finance lease liabilities - long-term	264	39
Total finance lease liabilities	$ 283	$ 56
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	12 years	12 years
Finance leases	31 years	5 years
Weighted average discount rate
Operating leases	4.50%	4.20%
Finance leases	5.70%	4.40%
Affiliated Entity
Leases, Operating [Abstract]
Operating lease right-of-use assets, net	$ 42	$ 47
Operating lease liabilities - current	5	5
Operating lease liabilities - long-term	$ 38	$ 43