LEASES - SUPPLEMENTAL STATEMENTS OF FINANCIAL POSITION (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Operating lease right-of-use assets, net	$ 713	$ 771
Operating lease liabilities - current	94	86
Operating lease liabilities - long-term	689	770
Total operating lease liabilities	$ 783	$ 856
Weighted average remaining lease term	13 years	14 years
Weighted average discount rate (percent)	4.30%	4.32%