ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) T
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)		$ (9.2)
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	$ 200.1	$ 206.8
Accrual for Environmental Loss Contingencies	2.9	2.8
Mine Reclamation and Closing Liability, current and noncurrent	197.2	204.0
U.S. Iron Ore [Member]
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	(26.2)
Asia Pacific Iron Ore [Member]
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	10.1
Previously Owned Or Operating Facilities [Member] | LTV Steel Mining Company [Member]
Site Contingency [Line Items]
Mine Reclamation and Closing Liability, current and noncurrent	$ 28.6	25.5
Selenium [Domain]
Site Contingency [Line Items]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)		$ (29.6)