PENSION AND OTHER POSTRETIREMENT BENEFITS - RETIREMENT SAVINGS PLAN (Details) - CAD CAD in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Canada
Pension and Other Postretirement Benefit Disclosures
Percent of match	5.00%
Total contributions by the Company	CAD 14	CAD 0	CAD 0
Canada | Pension
Pension and Other Postretirement Benefit Disclosures
Total contributions by the Company	161	74
Canada | OPEB
Pension and Other Postretirement Benefit Disclosures
Total contributions by the Company	CAD 10	6
United States
Pension and Other Postretirement Benefit Disclosures
Percent of match	6.00%
Total contributions by the Company	CAD 31	13	CAD 15
United States | Pension
Pension and Other Postretirement Benefit Disclosures
Total contributions by the Company	57	28
United States | OPEB
Pension and Other Postretirement Benefit Disclosures
Total contributions by the Company	CAD 1	CAD 3