ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Activity affecting the asset retirement and mine closing liability:
Balance at the beginning of the period	$ 125,701	$ 123,685
Accretion expense	3,793	3,769	$ 3,192
Payments	(1,046)	(379)
Allocation of liability associated with acquisitions, mine development and change in assumptions	2,152	(1,374)
Balance at the end of the period	$ 130,600	$ 125,701	$ 123,685