INCOME AND MINING TAXES - Tax Cuts and Jobs Act (the "Act") and Tax Reform (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			35.00%	34.00%	34.00%
Tax Cuts And Jobs Act Of 2017 Incomplete Accounting Change In Tax Rate Deferred Tax Liability Provisional Income Tax Expense			$ 4.2
Tax Cuts And Jobs Act Of 2017 Incomplete Accounting Change In Tax Rate Deferred Tax Asset Provisional Income Tax Expense			23.7
Tax Cuts And Jobs Act Of 2017 Income Tax Expense Benefit			$ 0.0
Argentina
Income Taxes
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			35.00%
Argentinian Tax Cuts Incomplete Accounting Change In Tax Rate Deferred Tax Liability Provisional Income Tax Expense			$ 3.9
Argentinian Tax Cuts Incomplete Accounting Change In Tax Rate Deferred Tax Assets Provisional Income Tax Expense			$ 12.4
Forecast
Income Taxes
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%
Forecast | Argentina
Income Taxes
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	25.00%