ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Environmental Remediation Obligations [Abstract]
Total environmental and mine closure obligations	$ 218.2	$ 170.8

Environmental Loss Contingency And Mine Reclamation And Closing Liability Current And Noncurrent

No definition available.

Name:	clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent
Namespace Prefix:	clf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant