PENSION AND OTHER POSTRETIREMENT BENEFITS - Actuarial Assumptions (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension | Canada
Projected benefit obligation
Discount rate	4.60%	5.10%	3.20%
Rate of salary increase	3.00%	2.90%	2.90%
Net periodic benefit cost
Discount rate	5.30%	3.20%	2.60%
Rate of return on plan assets	6.50%	6.60%	6.20%
Rate of salary increase	2.90%	2.90%	2.30%
Pension | US
Projected benefit obligation
Discount rate	4.70%	4.90%	2.60%
Rate of salary increase	2.60%	2.80%	2.80%
Cash balance interest credit rate	4.50%	4.30%	4.30%
Net periodic benefit cost
Discount rate	4.90%	2.60%	2.20%
Rate of return on plan assets	7.40%	7.40%	7.30%
Rate of salary increase	2.80%	2.80%	2.70%
Cash balance interest credit rate	4.30%	4.30%	4.30%
OPEB | Canada
Projected benefit obligation
Discount rate	4.60%	5.30%	3.20%
Net periodic benefit cost
Discount rate	5.30%	3.20%	2.60%
OPEB | US
Projected benefit obligation
Discount rate	4.70%	4.90%	2.40%
Net periodic benefit cost
Discount rate	4.90%	2.40%	2.00%
Rate of return on plan assets	5.90%	6.00%	6.00%