LEASES - Supplemental Statements of Financial Position (Details) - CAD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Operating lease right-of-use assets, net	$ 745	$ 771
Operating lease liabilities - current	67	86
Operating lease liabilities - long-term	757	770
Total operating lease liabilities	$ 824	$ 856
Weighted average remaining lease term	14 years	14 years
Weighted average discount rate	4.30%	4.30%