ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) T	Dec. 31, 2015 USD ($)
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	$ 9.2	$ (45.2)
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) | T	115,000,000
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent	$ 206.8	234.0
Payments for Environmental Liabilities	2.4	2.6
Accrual for Environmental Loss Contingencies	2.8	3.6
Mine Reclamation and Closing Liability, current and noncurrent	204.0	230.4
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member]
Site Contingency [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	25.5	$ 24.1
Selenium [Domain]
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	$ 29.6