Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTIES (Narrative) (Details)

v2.4.1.9
RELATED PARTIES (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Facility
Dec. 31, 2014
Segment Reporting Information [Line Items]    
Due from Related Parties, Current 61.2us-gaap_DueFromRelatedPartiesCurrent $ 127.6us-gaap_DueFromRelatedPartiesCurrent
Due to Related Parties, Current 0.1us-gaap_DueToRelatedPartiesCurrent $ 11.8us-gaap_DueToRelatedPartiesCurrent
U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number of mines (in number of facilities) 5clf_NumberOfMines
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
 
Joint Venture Partners [Member] | U.S. Iron Ore [Member]    
Segment Reporting Information [Line Items]    
Number of mines (in number of facilities) 3clf_NumberOfMines
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_CoVenturerMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember