Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Fair Values of Pension Plan Assets) (Details)

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Fair Values of Pension Plan Assets) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Reserve For Investment Commitments $ 55.5clf_ReserveForInvestmentCommitments $ 45.0clf_ReserveForInvestmentCommitments    
Fair value of plan assets 205.8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets   188.6us-gaap_DefinedBenefitPlanFairValueOfPlanAssets 200.5us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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