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INVENTORIES (Narrative) (Details) - USD ($) | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Inventory [Line Items] |  |
Impairment Charges | $ (18,900,000) | $ 0
LCM inventory adjustment |  | $ 0
Finished Goods [Domain] |  |
Inventory [Line Items] |  |
Impairment Charges | 1,400,000 |
LCM inventory adjustment | 13,000,000 |
Work in Process [Domain] |  |
Inventory [Line Items] |  |
Impairment Charges | 13,200,000 |
LCM inventory adjustment | $ 9,100,000 |

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- Definition Impairment charges including, but not limited to, impairment charges related to intangible and tangible assets, investment, and goodwill. + References No definition available. + Details
Name: | clf_ImpairmentCharges
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition LCM inventory adjustment + References No definition available. + Details
Name: | clf_LCMinventoryadjustment
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_InventoryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | clf_InventorytypeAxis=clf_FinishedGoodsDomain
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_InventorytypeAxis=clf_WorkinProcessDomain
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |