INCOME TAXES (Components of Earnings Before Taxes and Tax Effect) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. (loss) income	$ (4.1)	$ 31.7	$ (80.6)
Foreign income	75.3	52.9	158.2
Earnings before income taxes	$ 71.2	$ 84.6	$ 77.6
U.S. federal statutory rate	21.00%	21.00%	21.00%
Computed tax at the U.S. federal statutory rate of 21%	$ 15.0	$ 17.8	$ 16.3
Foreign income rate differential, mining, and withholding taxes, net of U.S. federal deduction	9.3	10.2	0.9
Percentage depletion in excess of basis	(4.4)	(5.8)	(4.7)
Other domestic tax reserves, net of reversals	(9.7)	0.2	1.5
State income taxes, net of federal income tax benefit	(0.9)	0.0	2.1
Change in valuation allowance on deferred tax asset	2.4	0.2	(5.7)
Interest expense recognition differences	(3.5)	(3.5)	(3.6)
GILTI	3.8	1.0	1.0
Tax Cuts and Jobs Act of 2017	0.0	1.6	(3.0)
Tax on repatriated amounts	0.0	0.9	3.4
Transfer pricing settlement with taxing authorities	0.0	0.0	2.2
Other, net	(0.3)	(0.5)	(1.6)
Total provision for income taxes	$ 11.7	$ 22.1	$ 8.8
Effective tax rate	16.00%	26.00%	11.00%