LEASES - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Lessee, Lease, Description [Line Items]
Operating lease, expense	$ 28	$ 56
Operating lease, payments	$ 30	$ 61
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term		6 months
Lessor, operating lease, remaining lease term		1 month
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term		29 years
Lessor, operating lease, remaining lease term		24 years