| GOODWILL AND OTHER INTANGIBLE ASSETS AND LIABILITIES (Schedule Of Goodwill) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 
|---|---|---|
| Mar. 31, 2015 | Dec. 31, 2014 | |
| Goodwill [Roll Forward] | ||
| Beginning Balance | $ 2.0us-gaap_Goodwill | $ 74.5us-gaap_Goodwill | 
| Arising in business combinations | 0us-gaap_GoodwillAcquiredDuringPeriod | 0us-gaap_GoodwillAcquiredDuringPeriod | 
| Impairment | 0us-gaap_GoodwillImpairmentLoss | (73.5)us-gaap_GoodwillImpairmentLoss | 
| Impact of foreign currency translation | 0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments | 1.0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments | 
| Ending Balance | 2.0us-gaap_Goodwill | 2.0us-gaap_Goodwill | 
| Accumulated goodwill impairment loss | (73.5)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | (73.5)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss | 
| U.S. Iron Ore [Member] | ||
| Goodwill [Roll Forward] | ||
| Beginning Balance | 2.0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 2.0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Arising in business combinations | 0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Impairment | 0us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 0us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Impact of foreign currency translation | 0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Ending Balance | 2.0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 2.0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Accumulated goodwill impairment loss | 0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 
| Asia Pacific Iron Ore [Member] | ||
| Goodwill [Roll Forward] | ||
| Beginning Balance | 0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 72.5us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
| Arising in business combinations | 0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 0us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
| Impairment | 0us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | (73.5)us-gaap_GoodwillImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
| Impact of foreign currency translation | 0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 1.0us-gaap_GoodwillTranslationAndPurchaseAccountingAdjustments / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
| Ending Balance | 0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 0us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
| Accumulated goodwill impairment loss | $ (73.5)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | $ (73.5)us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 
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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of increase (decrease) from foreign currency translation adjustments and purchase accounting adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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