PENSION AND OTHER POSTRETIREMENT BENEFITS - RETIREMENT SAVINGS PLAN (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Canada
Pension and Other Postretirement Benefit Disclosures
Percent of match (percent)	2.50%
Total contributions by the Company	$ 0	$ 4	$ 13
United States
Pension and Other Postretirement Benefit Disclosures
Percent of match (percent)	6.00%
Total contributions by the Company	$ 27