PROPERTY, PLANT AND EQUIPMENT - Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mining Equipment And Processing Facilities
Property, Plant and Equipment
Equipment under capital leases	$ 140.9
Accumulated amortization related to capital leases	55.6	$ 34.2	$ 7.1
Amortization expense related to capital leases	24.9	27.2	$ 5.7
Mine development costs
Property, Plant and Equipment
Capitalized mine development costs	$ 0.0	$ 0.0