PENSION AND OTHER POSTRETIREMENT BENEFITS - ACTUARIAL ASSUMPTIONS (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension | Canada
Projected benefit obligation
Discount rate	2.60%	3.00%	3.80%
Rate of salary increase	2.30%	3.20%	3.20%
Net periodic benefit cost
Discount rate	3.00%	3.80%	3.60%
Rate of return on plan assets	6.80%	7.00%	6.80%
Rate of salary increase	3.20%	3.20%	3.20%
Pension | United States
Projected benefit obligation
Discount rate	2.20%	3.00%	3.90%
Rate of salary increase	2.70%	2.90%	2.80%
Cash balance interest credit rate	4.30%	4.50%	4.50%
Net periodic benefit cost
Discount rate	3.00%	3.90%	3.40%
Rate of return on plan assets	7.90%	8.00%	7.40%
Rate of salary increase	2.90%	2.90%	2.90%
Cash balance interest credit rate	4.50%	4.50%	4.50%
OPEB | Canada
Projected benefit obligation
Discount rate	2.60%	3.10%	3.80%
Net periodic benefit cost
Discount rate	3.10%	3.80%	3.60%
OPEB | United States
Projected benefit obligation
Discount rate	2.00%	2.80%	4.00%
Net periodic benefit cost
Discount rate	2.80%	4.00%	3.30%
Rate of return on plan assets	6.70%	6.70%	5.70%