LEASES - Schedule of future maturities of lease liabilities (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Finance
Remainder of 2019	$ 13,566
2020	20,209
2021	6,541
2022	129
2023	108
Thereafter	5
Total minimum lease payments	40,558
Less amount representing interest	2,260
Present value of minimum lease payments	38,298
Operating
Remainder of 2019	19,751
2020	23,921
2021	23,134
2022	13,341
2023	6,504
Thereafter	22,113
Total minimum lease payments	108,764
Less amount representing interest	18,564
Present value of minimum lease payments	90,200
Mining equipment
Finance
Remainder of 2019	2,774
2020	3,699
2021	2,157
2022	0
2023	0
Thereafter	0
Total minimum lease payments	8,630
Operating
Remainder of 2019	1,237
2020	627
2021	0
2022	0
2023	0
Thereafter	0
Total minimum lease payments	$ 1,864