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PROPERTY, PLANT AND EQUIPMENT (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Property, Plant and Equipment [Line Items] |  |  |  |
Depreciation And Depletion | $ 29.5 | $ 68.4 | $ 58.2 | $ 136.1

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- Definition 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. + References No definition available. + Details
Name: | clf_DepreciationAndDepletionExpense
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration