PENSION AND OTHER POSTRETIREMENT BENEFITS - ACTUARIAL ASSUMPTIONS (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension | Canada
Projected benefit obligations (as percent)
Discount rate	3.00%	3.80%	3.60%
Rate of salary increase	3.20%	3.20%	3.20%
Net benefit cost (as percent)
Discount rate	3.80%	3.60%	4.00%
Expected rate of return on plan assets	7.00%	6.80%	6.50%
Rate of salary increase	3.20%	3.20%	3.70%
Pension | United States
Projected benefit obligations (as percent)
Discount rate	3.00%	3.90%	3.50%
Rate of salary increase	2.90%	2.80%	3.10%
Net benefit cost (as percent)
Discount rate	3.90%	3.40%	4.00%
Expected rate of return on plan assets	8.00%	7.40%	7.20%
Rate of salary increase	2.90%	2.90%	3.30%
OPEB | Canada
Projected benefit obligations (as percent)
Discount rate	3.10%	3.80%	3.60%
Net benefit cost (as percent)
Discount rate	3.80%	3.60%	4.00%
OPEB | United States
Projected benefit obligations (as percent)
Discount rate	2.80%	4.00%	3.50%
Net benefit cost (as percent)
Discount rate	4.00%	3.30%	4.00%
Expected rate of return on plan assets	6.70%	5.70%	6.00%