SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Tangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment
Coal reserves not subject to depletion	$ 30.7	$ 53.2
Mine development costs
Capitalized mine development costs	$ 5.9	$ 7.0
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	29 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	2 years
Buildings, office equipment and improvements | Maximum
Property, Plant and Equipment
Estimated useful life	29 years
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