RETIREMENT PLANS - Rate Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
One-percentage change in assumed health care cost trend rates
Accumulated postretirement benefit obligation, At trend	$ 4,900	$ 4,013
Accumulated postretirement benefit obligation, At trend +1%	5,242	4,305
Impact of Accumulated postretirement benefit obligation, At trend +1%	$ 342	$ 292
Impact of Accumulated postretirement benefit obligation, At trend +1% (as a percent)	7.00%	7.30%
Accumulated postretirement benefit obligation, At trend -1%	$ 4,597	$ 3,755
Impact of Accumulated postretirement benefit obligation, At trend -1%	$ (303)	$ (258)
Impact of Accumulated postretirement benefit obligation, At trend -1% (as a percent)	(6.20%)	(6.40%)
Service cost plus interest cost, At trend	$ 308	$ 308
Service cost plus interest cost, At Trend +1%	337	338
Impact of Service cost plus interest cost, At trend +1%	$ 29	$ 30
Impact of Service cost plus interest cost, At trend +1% (as a percent)	9.40%	9.70%
Service cost plus interest cost, At Trend -1%	$ 284	$ 282
Impact of Service cost plus interest cost, At trend -1%	$ (24)	$ (26)
Impact of Service cost plus interest cost, At trend -1% (as a percent)	(7.80%)	(8.40%)
Pension Plans
Weighted-average rates assumed in the actuarial calculations for the pension plan
Discount rate for benefit obligation (as percent)	4.20%	4.70%	4.20%
Discount rate for benefit cost (as percent)	4.50%	4.30%	4.60%
Annual salary increase (as a percent)	6.50%	6.50%	6.50%
Expected long-term return on plan assets (as a percent)	6.10%	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)	3.50%	4.50%	3.90%
Discount rate for benefit cost (as percent)	4.50%	3.90%	4.30%
Postretirement Benefits \| Medical benefit plan
Health care cost trend rates
2019 Trend rate	6.00%
Ultimate rate	4.00%
Postretirement Benefits \| Dental benefit plan
Health care cost trend rates
2019 Trend rate	4.00%
Ultimate rate	4.00%

For defined benefit pension plans, the actuarial present value of benefits decreased one percentage point attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date.

No definition available.