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EARNINGS PER SHARE EARNINGS PER SHARE (Narrative) (Details) shares in Millions | 3 Months Ended
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Mar. 31, 2017 shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] |
Antidilutive Incremental Common Shares Excluded and Attributable to Share-based Payment Arrangements | 4.6

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- Definition Antidilutive Incremental Common Shares Excluded and Attributable to Share-based Payment Arrangements + References No definition available. + Details
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Namespace Prefix: | clf_
Data Type: | xbrli:sharesItemType
Balance Type: | na
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_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration