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SEGMENT REPORTING (Narrative) (Details) | 9 Months Ended
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Sep. 30, 2015 Facility
U.S. Iron Ore [Member] |
Segment Reporting Information [Line Items] |
Number of mines (in number of facilities) | 5
Asia Pacific Iron Ore [Member] |
Segment Reporting Information [Line Items] |
Number of mines (in number of facilities) | 1

X
- Definition Number of mines + References No definition available. + Details
Name: | clf_NumberOfMines
Namespace Prefix: | clf_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=clf_USIronOreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=clf_AsiaPacificIronOreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |