SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Other Current Assets & Property, Plant, & Equipment) (Details) $ in Millions		12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) extension_period	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Other Current Assets
Assets held-for-sale	$ 7.2	$ 7.2	$ 8.0
Tax refund from settlement			(17.5)
Capitalized computer software costs	$ 16.0	$ 16.0	23.6
Period of amortization of capitalized computer software costs		5 years
Amortization of capitalized computer software costs		$ 6.6	7.3	$ 4.7
Land improvements | Minimum
Other Current Assets
Estimated useful life		10 years
Land improvements | Maximum
Other Current Assets
Estimated useful life		25 years
Buildings and structures | Minimum
Other Current Assets
Estimated useful life	10 years	10 years
Buildings and structures | Maximum
Other Current Assets
Estimated useful life	30 years	30 years
Leasehold and building improvements | Minimum
Other Current Assets
Estimated useful life		5 years
Leasehold and building improvements | Maximum
Other Current Assets
Estimated useful life		40 years
Vehicles | Minimum
Other Current Assets
Estimated useful life		3 years
Vehicles | Maximum
Other Current Assets
Estimated useful life		10 years
Other mining and production machinery and equipment | Minimum
Other Current Assets
Estimated useful life		3 years
Other mining and production machinery and equipment | Maximum
Other Current Assets
Estimated useful life		50 years
Office furniture and equipment | Minimum
Other Current Assets
Estimated useful life		3 years
Office furniture and equipment | Maximum
Other Current Assets
Estimated useful life		10 years
Mineral interests | Minimum
Other Current Assets
Estimated useful life		20 years
Mineral interests | Maximum
Other Current Assets
Estimated useful life		99 years
Cote Blanche Mine
Other Current Assets
Number of renewal periods (in extension periods) | extension_period		2
Lease renewal period		25 years
Leased Mineral Interests
Other Current Assets
Estimated useful life	91 years
Owned Mineral Properties
Other Current Assets
Estimated useful life	37 years
Typical maximum life leasehold and building improvements | Minimum
Other Current Assets
Estimated useful life	5 years
Typical maximum life leasehold and building improvements | Maximum
Other Current Assets
Estimated useful life	20 years
Other current assets | U.S. taxing authorities
Other Current Assets
Tax refund from settlement	$ 58.3	$ 58.3	81.5
Water rights | Other current assets
Other Current Assets
Assets held-for-sale	$ 5.2	$ 5.2	$ 5.2