SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Tangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Plant and equipment assets, other than preparation plants and processing facilities | Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Plant and equipment assets, other than preparation plants and processing facilities | Maximum
Property, Plant and Equipment
Estimated useful life	22 years
Mining Equipment | Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Mining Equipment | Maximum
Property, Plant and Equipment
Estimated useful life	22 years
Buildings, Office Equipment And Improvements | Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Buildings, Office Equipment And Improvements | Maximum
Property, Plant and Equipment
Estimated useful life	24 years
Land And Mineral Rights
Property, Plant and Equipment
Coal reserves not subject to depletion	$ 34.5	$ 34.4
Land And Mineral Rights | Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Land And Mineral Rights | Maximum
Property, Plant and Equipment
Estimated useful life	22 years