PROPERTY, PLANT AND EQUIPMENT - Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mining Equipment And Processing Facilities
Property, Plant and Equipment
Equipment under capital leases	$ 144.5
Accumulated amortization related to capital leases	34.2	$ 7.1	$ 5.6
Amortization expense related to capital leases	27.2	5.7	$ 1.6
Mine development costs
Property, Plant and Equipment
Capitalized mine development costs	$ 0.0	$ 5.9