LEASES - NARRATIVE (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease, expense	$ 107	$ 113
Operating lease, payments	$ 133	$ 123
Minimum
Lessee, Lease, Description [Line Items]
Remaining lease term	1 month
Lessor, operating lease, remaining lease term	1 month
Maximum
Lessee, Lease, Description [Line Items]
Remaining lease term	35 years
Lessor, operating lease, remaining lease term	23 years