LEASES - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Operating lease, expense	$ 28	$ 84
Operating lease, payments	$ 31	$ 92
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term		3 months
Lessor, operating lease, remaining lease term		1 month
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, remaining lease term		28 years
Lessor, operating lease, remaining lease term		24 years