LEASES - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 CAD ($)
Lessee, Lease, Description [Line Items]
Operating lease, expense	$ 28
Operating lease, payments	$ 31
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term	1 month
Lessor, operating lease, remaining lease term	9 months
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term	29 years
Lessor, operating lease, remaining lease term	24 years