COMMITMENTS AND CONTINGENCIES - Schedule of Long-term Purchase Commitment (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Long-term Purchase Commitment [Line Items]
Total amounts committed	$ 69,688
2020	10,215
2021-2023	54,600
2024	2,595
After 5 years	2,278
Long-term debt (includes interest)
Long-term Purchase Commitment [Line Items]
Total amounts committed	3,922
2020	1,689
2021-2023	1,480
2024	335
After 5 years	418
Finance leases (includes interest)
Long-term Purchase Commitment [Line Items]
Total amounts committed	13,580
2020	4,610
2021-2023	8,449
2024	521
After 5 years	0
Future equipment purchases
Long-term Purchase Commitment [Line Items]
Total amounts committed	40,910
2020	1,750
2021-2023	39,160
2024	0
After 5 years	0
Equipment service contracts
Long-term Purchase Commitment [Line Items]
Total amounts committed	10,096
2020	1,847
2021-2023	4,655
2024	1,734
After 5 years	1,860
Operating leases
Long-term Purchase Commitment [Line Items]
Total amounts committed	1,180
2020	319
2021-2023	856
2024	5
After 5 years	$ 0