ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) T	Mar. 31, 2017 USD ($)
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	$ 9.2
Reserve increase (decrease), tons | T	115,000,000
Selenium [Domain]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	$ (29.6)
Eastern Canadian Iron Ore [Member] | Owned Or Operating Facilities [Member]
Mine Reclamation and Closing Liability, current and noncurrent	$ 206.8	$ 211.0