PROPERTY, PLANT AND EQUIPMENT (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation and depletion	$ 21.3	$ 22.6
Impairment Charges, CIP	$ (2.6)

The aggregate expense recognized in the current period that allocates the cost of tangible assets or depleting assets to periods that benefit from use of the assets.

No definition available.

Name:	clf_DepreciationAndDepletionExpense
Namespace Prefix:	clf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration