DEBT (Standby Letters of Credit) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Risk management insurance	$ 49,310	$ 44,618
Reclamation/restoration requirements	223,872	$ 226,594	$ 226,565
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Risk management insurance	34,111
Reclamation/restoration requirements	5,427
Total	$ 39,538