INCOME TAXES - Changes to valuation allowance (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation allowance
Change in valuation allowance		$ 7,000	$ 4,600
Balance at Beginning of Period	$ 125,202	129,794	125,202	$ 90,477
Additions		6,873	11,514	35,547
Deductions		(13,899)	(6,922)	(822)
Balance at End of Period		$ 122,768	$ 129,794	$ 125,202
Special mining duty (as a percent)		7.50%
Mexico
Valuation allowance
Deductions related to development type costs	$ 0
Special mining duty (as a percent)	7.50%