PENSION AND OTHER POSTRETIREMENT BENEFITS - ACTUARIAL ASSUMPTIONS (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension | Canada
Projected benefit obligations
Discount rate	3.60%	4.00%	4.20%
Rate of salary increase	3.20%	3.70%	3.60%
Net benefit cost
Discount rate	4.00%	4.20%	4.00%
Rate of return on plan assets	6.50%	6.50%	4.40%
Rate of salary increase	3.70%	3.60%	2.50%
Pension | United States
Projected benefit obligations
Discount rate	3.50%	4.00%	4.10%
Rate of salary increase	3.10%	3.30%	3.30%
Net benefit cost
Discount rate	4.00%	4.10%	3.70%
Rate of return on plan assets	7.20%	7.20%	7.10%
Rate of salary increase	3.30%	3.20%	4.00%
OPEB | Canada
Projected benefit obligations
Discount rate	3.60%	4.00%	4.20%
Net benefit cost
Discount rate	4.00%	4.20%	4.00%
Rate of return on plan assets
OPEB | United States
Projected benefit obligations
Discount rate	3.50%	3.60%	4.20%
Net benefit cost
Discount rate	4.00%	3.80%	3.90%
Rate of return on plan assets	6.00%	6.00%	6.00%