Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v2.4.1.9
STOCK COMPENSATION PLANS (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Agreement
Plan
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plans 2clf_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPlans    
Allocated Share-based Compensation Expense $ 23.7us-gaap_AllocatedShareBasedCompensationExpense $ 21.1us-gaap_AllocatedShareBasedCompensationExpense $ 20.6us-gaap_AllocatedShareBasedCompensationExpense
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense 8.3us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 7.4us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 7.2us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plan Year Agreements 3clf_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfPlanYearAgreements    
Number of performance shares granted 2,016,996us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized 18.3us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 2 years 6 months    
Option Indexed to Issuer's Equity, Strike Price $ 13.83us-gaap_OptionIndexedToIssuersEquityStrikePrice1    
2012 Equity Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares granted under the award 800,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanMember
   
Number of performance shares granted 400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanMember
   
Number of restricted shares granted 300,000us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanMember
   
2012 Equity Plan [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of shares that may be issued (in shares) 6,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Two Thousand Twelve Equity Plan and Amended Equity Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance/vesting period 3 years    
Payout rate, as a percentage of the original grant   23.00%clf_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodPayoutRate
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanandAmendedEquityPlanMember
50.00%clf_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodPayoutRate
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanandAmendedEquityPlanMember
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Available for Issuance Proposed to be added to the Plan 5,000,000clf_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofAdditionalSharesAvailableforIssuanceProposedtobeaddedtothePlan
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanandAmendedEquityPlanMember
   
Change in Control [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated Share-based Compensation Expense $ 11.7us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis
= clf_ChangeinControlMember
   
Two Thousand Fourteen to Two Thousand Sixteen Performance Period [Member] | Two Thousand Twelve Equity Plan and Amended Equity Plan [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Payout rate, as a percentage of the original grant 0.00%clf_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodPayoutRate
/ us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis
= clf_TwoThousandFourteentoTwoThousandSixteenPerformancePeriodMember
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanandAmendedEquityPlanMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Two Thousand Fourteen to Two Thousand Sixteen Performance Period [Member] | Two Thousand Twelve Equity Plan and Amended Equity Plan [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Payout rate, as a percentage of the original grant 200.00%clf_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodPayoutRate
/ us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis
= clf_TwoThousandFourteentoTwoThousandSixteenPerformancePeriodMember
/ us-gaap_PlanNameAxis
= clf_TwoThousandTwelveEquityPlanandAmendedEquityPlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember