DEBT (Standby Letters of Credit) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Risk management insurance	$ 58,216	$ 49,310
Reclamation/restoration requirements	218,117	$ 223,872	$ 226,594
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Risk management insurance	38,111
Reclamation/restoration requirements	5,128
Total	$ 43,239