LEASES - SUPPLEMENTAL STATEMENTS OF FINANCIAL POSITION (Details) - CAD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Supplemental Statements Of Financial Position
Operating lease right-of-use assets, net	$ 645	$ 708
Operating lease liabilities - current	92	80
Operating lease liabilities - long-term	612	681
Total	704	761
Finance lease right-of-use assets, net	49	57
Finance lease liabilities - current	13	11
Finance lease liabilities - long-term	33	42
Total finance lease liabilities	$ 46	$ 53
Operating leases - Weighted average remaining lease term	12 years	13 years
Finance leases - Weighted average remaining lease term	7 years	7 years
Operating leases - Weighted average discount rate	4.10%	4.10%
Finance leases - Weighted average discount rate	3.80%	3.80%
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 (Note 17)	Accounts payable and other (Note 17)
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other long-term liabilities	Other long-term liabilities
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 18)	Current portion of long-term debt (Note 18)
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other long-term liabilities	Other long-term liabilities
Operating lease right-of-use assets	$ 645	$ 708
Affiliated Entity
Supplemental Statements Of Financial Position
Operating lease right-of-use assets, net	51	65
Operating lease liabilities - current	5	5
Operating lease liabilities - long-term	47	52
Operating lease right-of-use assets	$ 51	$ 65