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Annual report pursuant to Section 13 and 15(d)

SEGMENT REPORTING Segment Reporting (Summary of Assets by Segment) (Details)

v2.4.1.9
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
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets $ 3,164.0us-gaap_Assets $ 13,121.9us-gaap_Assets $ 13,574.9us-gaap_Assets
U.S. Iron Ore [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 1,598.3us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		1,671.6us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember 		1,735.1us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
Eastern Canadian Iron Ore [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 305.8us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		7,915.5us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember 		7,605.1us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_EasternCanadianIronOreMember
Asia Pacific Iron Ore [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 274.6us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		1,078.4us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember 		1,506.3us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember
North American Coal [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 274.2us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember 		1,841.8us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember 		1,877.8us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= clf_NorthAmericanCoalMember
All Other Segments [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 164.3us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember 		455.6us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember 		570.9us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_AllOtherSegmentsMember
Total Segment Assets [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets 2,617.2us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_OperatingSegmentsMember 		12,962.9us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_OperatingSegmentsMember 		13,295.2us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_OperatingSegmentsMember
Corporate [Member] Ìý Ìý Ìý
Segment Reporting Information [Line Items] Ìý Ìý Ìý
Assets $ 546.8us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember 		$ 159.0us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember 		$ 279.7us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateMember