Annual report pursuant to Section 13 and 15(d)

PROPERTY, PLANT AND EQUIPMENT (Book Value of Land and Mineral Rights Disclosure) (Details)

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PROPERTY, PLANT AND EQUIPMENT (Book Value of Land and Mineral Rights Disclosure) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Abstract]    
Land Rights $ 12.4 $ 12.4
Mineral Rights [Abstract]    
Mineral Properties, Gross 537.2 537.2
Mineral Properties, Accumulated Depletion (126.5) (119.1)
Mineral Properties, Net $ 410.7 $ 418.1