ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Jun. 30, 2018
Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent	$ 171.3	$ 176.0
U.S. Iron Ore [Member]
Accrual for Environmental Loss Contingencies, Period Increase (Decrease)	$ (26.2)

Increase (decrease) in the accrual for environmental loss contingencies during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 5.Y.Q2)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

Name:	us-gaap_AccrualForEnvironmentalLossContingenciesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration