RETIREMENT PLANS - Rate Assumptions (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Weighted-average rates assumed in the actuarial calculations for the pension plan
Discount rate for benefit obligation (as percent)	2.60%	4.20%	4.70%
Discount rate for benefit cost (as percent)	3.50%	4.50%	4.30%
Annual salary increase (as a percent)		6.50%	6.50%
Expected long-term return on plan assets (as a percent)	5.10%	6.10%	6.10%
Postretirement Benefits
Weighted-average rates assumed in the actuarial calculations for the pension plan
Discount rate for benefit obligation (as percent)	2.50%	3.50%	4.50%
Discount rate for benefit cost (as percent)	3.50%	4.50%	3.90%
Postretirement Benefits | Medical benefit plan
Health care cost trend rates
Trend rate	6.00%
Ultimate rate	4.00%
Postretirement Benefits | Dental benefit plan
Health care cost trend rates
Ultimate rate	4.00%