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SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Segment Reporting Information [Line Items] |  |  |
Assets | $ 3,164.0 us-gaap_Assets | $ 13,121.9 us-gaap_Assets | $ 13,574.9 us-gaap_Assets
U.S. Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 1,598.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 1,671.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 1,735.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember
Eastern Canadian Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 305.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | 7,915.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | 7,605.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember
Asia Pacific Iron Ore [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 274.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 1,078.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 1,506.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember
North American Coal [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 274.2 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 1,841.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | 1,877.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember
All Other Segments [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 164.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 455.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | 570.9 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember
Total Segment Assets [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | 2,617.2 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember | 12,962.9 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember | 13,295.2 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_OperatingSegmentsMember
Corporate [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Assets | $ 546.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember | $ 159.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember | $ 279.7 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember

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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 + Details
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