LEASES - SUPPLEMENTAL STATEMENTS OF FINANCIAL POSITION (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease right-of-use assets, net	$ 708	$ 713
Operating lease liabilities - current	80	94
Operating lease liabilities - long-term	681	689
Total operating lease liabilities	761	783
Finance lease right-of-use assets, net	124	89
Finance lease liabilities - current	17	16
Finance lease liabilities - long-term	98	78
Total finance lease liabilities	$ 115	$ 94
Operating leases - Weighted average remaining lease term	13 years	13 years
Finance leases - Weighted average remaining lease term	22 years	23 years
Operating leases - Weighted average discount rate	4.10%	4.30%
Finance leases - Weighted average discount rate	2.90%	3.60%
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:DeferredCostsAndOtherAssets	us-gaap:DeferredCostsAndOtherAssets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndOtherAccruedLiabilitiesCurrent	us-gaap:AccountsPayableAndOtherAccruedLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	us-gaap:DeferredCostsAndOtherAssets	us-gaap:DeferredCostsAndOtherAssets
Finance Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccountsPayableAndOtherAccruedLiabilitiesCurrent	us-gaap:AccountsPayableAndOtherAccruedLiabilitiesCurrent
Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent