ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Activity affecting the asset retirement and mine closing liability:
Balance at the beginning of the period	$ 93,140	$ 93,140	$ 82,898
Accretion expense	3,192	2,730	3,004
Payments	(519)	(1,134)
Assumption of existing liability	31,372	6,042
Disposition		(5,246)
Allocation of liability associated with acquisition, mine development and change in assumptions	(3,500)	7,850
Balance at the end of the period	$ 123,685	$ 93,140	$ 93,140