DEBT (Standby Letters of Credit) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Risk management insurance	$ 68,912	$ 58,216
Reclamation/restoration requirements	225,726	$ 218,117	$ 223,872
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Risk management insurance	38,111
Reclamation/restoration requirements	6,868
Total	$ 44,979