ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Activity affecting the asset retirement and mine closing liability:
Balance at the beginning of the period	$ 123,685	$ 93,140
Accretion expense	3,769	3,192	$ 2,730
Payments	(379)	(519)
Assumption of existing liability		31,372
Allocation of liability associated with acquisition, mine development and change in assumptions	1,374	3,500
Balance at the end of the period	$ 125,701	$ 123,685	$ 93,140