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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

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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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- References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration