PROPERTY, PLANT AND EQUIPMENT (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Depreciation and depletion	$ 21.0	$ 25.6	$ 64.8	$ 85.1

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