RETIREMENT PLANS - Rate Assumptions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
One-percentage change in assumed health care cost trend rates
Accumulated postretirement benefit obligation, At trend	$ 5,308,000	$ 5,108,000
Accumulated postretirement benefit obligation, At trend +1%	5,723,000	5,487,000
Impact of Accumulated postretirement benefit obligation, At trend +1%	$ 415,000	$ 379,000
Impact of Accumulated postretirement benefit obligation, At trend +1% (as a percent)	7.80%	7.40%
Accumulated postretirement benefit obligation, At trend -1%	$ 4,944,000	$ 4,775,000
Impact of Accumulated postretirement benefit obligation, At trend -1%	$ (364,000)	$ (333,000)
Impact of Accumulated postretirement benefit obligation, At trend -1% (as a percent)	(6.90%)	(6.50%)
Service cost plus interest cost, At trend	$ 389,000	$ 194,000
Service cost plus interest cost, At Trend +1%	429,000	214,000
Impact of Service cost plus interest cost, At trend +1%	40,000	20,000
Impact of Service cost plus interest cost, At trend +1% (as a percent)	10.3	10.3
Service cost plus interest cost, At Trend -1%	355,000	177,000
Impact of Service cost plus interest cost, At trend -1%	$ (34,000)	$ (17,000)
Impact of Service cost plus interest cost, At trend -1% (as a percent)	(8.70%)	(8.80%)
Pension Plans
Weighted-average rates assumed in the actuarial calculations for the pension plan
Discount rate (as a percent)	4.60%	4.30%	5.80%
Annual salary increase (as a percent)	6.50%	6.50%	6.50%
Expected long-term return on plan assets (as a percent)	6.10%	6.30%	6.50%
Postretirement Benefits
Weighted-average rates assumed in the actuarial calculations for the pension plan
Discount rate (as a percent)	4.30%	3.90%
Postretirement Benefits | Medical benefit plan
Health care cost trend rates
2018 Trend rate	5.30%
2018 Ultimate rate	4.50%
Postretirement Benefits | Dental benefit plan
Health care cost trend rates
2018 Trend rate	4.00%
2018 Ultimate rate	2.00%