Annual report pursuant to Section 13 and 15(d)

INCOME TAXES INCOME TAXES (Schedule of Effective Rate Reconciliation) (Details)

v2.4.1.9
INCOME TAXES INCOME TAXES (Schedule of Effective Rate Reconciliation) (Details)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
Dec. 31, 2014
USD ($)
Dec. 31, 2014
Dec. 31, 2013
USD ($)
Dec. 31, 2013
Dec. 31, 2012
USD ($)
Dec. 31, 2012
Income Tax Contingency [Line Items]              
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount   $ (3,361.3)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate   $ 171.3us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate   $ (175.6)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate  
Effective Income Tax Rate Reconciliation, Percent   35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Foreign Currency Transaction Gain, before Tax   (4.1)us-gaap_ForeignCurrencyTransactionGainBeforeTax   (2.6)us-gaap_ForeignCurrencyTransactionGainBeforeTax   62.3us-gaap_ForeignCurrencyTransactionGainBeforeTax  
Foreign Currency Transaction Gain Before Tax Percent   0.00%clf_ForeignCurrencyTransactionGainBeforeTaxPercent 0.00%clf_ForeignCurrencyTransactionGainBeforeTaxPercent (0.50%)clf_ForeignCurrencyTransactionGainBeforeTaxPercent (0.50%)clf_ForeignCurrencyTransactionGainBeforeTaxPercent (12.40%)clf_ForeignCurrencyTransactionGainBeforeTaxPercent (12.40%)clf_ForeignCurrencyTransactionGainBeforeTaxPercent
Non-taxable loss (income) related to noncontrolling interests   290.1clf_NonTaxableLossIncomeRelatedToNoncontrollingInterests   (1.5)clf_NonTaxableLossIncomeRelatedToNoncontrollingInterests   61.0clf_NonTaxableLossIncomeRelatedToNoncontrollingInterests  
Non-taxable loss (income) related to noncontrolling interests percent   (3.00%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent (3.00%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent (0.30%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent (0.30%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent (12.00%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent (12.00%)clf_Nontaxablelossincomerelatedtononcontrollinginterestspercent
Impact Of Tax Law Change   13.0clf_ImpactOfTaxLawChange   0clf_ImpactOfTaxLawChange   (357.1)clf_ImpactOfTaxLawChange  
Impact Of Tax Law Change Percent   (0.10%)clf_ImpactOfTaxLawChangePercent (0.10%)clf_ImpactOfTaxLawChangePercent 0.00%clf_ImpactOfTaxLawChangePercent 0.00%clf_ImpactOfTaxLawChangePercent 71.20%clf_ImpactOfTaxLawChangePercent 71.20%clf_ImpactOfTaxLawChangePercent
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion   (87.9)clf_IncomeTaxReconciliationTaxDeductionForPercentageDepletionInExcessOfCostDepletion   (97.6)clf_IncomeTaxReconciliationTaxDeductionForPercentageDepletionInExcessOfCostDepletion   (109.1)clf_IncomeTaxReconciliationTaxDeductionForPercentageDepletionInExcessOfCostDepletion  
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion percent   0.90%clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent 0.90%clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent (19.90%)clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent (19.90%)clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent 21.70%clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent 21.70%clf_IncomeTaxReconciliationTaxDeductionforpercentagedepletioninexcessofcostdepletionpercent
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount   592.0us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential   (10.2)us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential   65.2us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential  
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent   (6.20%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (6.20%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (2.10%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (2.10%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (13.00%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (13.00%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential
Income not subject to tax   (46.5)clf_IncomeNotSubjectToTax   (106.6)clf_IncomeNotSubjectToTax   (108.0)clf_IncomeNotSubjectToTax  
Income not subject to tax percent   0.50%clf_Incomenotsubjecttotaxpercent 0.50%clf_Incomenotsubjecttotaxpercent (21.80%)clf_Incomenotsubjecttotaxpercent (21.80%)clf_Incomenotsubjecttotaxpercent 21.50%clf_Incomenotsubjecttotaxpercent 21.50%clf_Incomenotsubjecttotaxpercent
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount   22.7us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses   20.5us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses   202.2us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses  
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent   (0.20%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses (0.20%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses 4.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses 4.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses (40.30%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses (40.30%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseImpairmentLosses
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount   (43.6)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes   5.6us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes   7.3us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes  
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent   0.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 0.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 1.10%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes 1.10%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Effective Income Tax Rate Reconciliation, Tax Settlement, Amount   (343.3)us-gaap_IncomeTaxReconciliationTaxSettlements   0us-gaap_IncomeTaxReconciliationTaxSettlements   0us-gaap_IncomeTaxReconciliationTaxSettlements  
Effective Income Tax Rate Reconciliation, Tax Settlement, Percent   3.60%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements 3.60%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements 0.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements
Income Tax Reconciliation Manufacturers Deduction   0clf_IncomeTaxReconciliationManufacturersDeduction   (7.9)clf_IncomeTaxReconciliationManufacturersDeduction   (4.7)clf_IncomeTaxReconciliationManufacturersDeduction  
Income Tax Reconciliation, Manufacturers Deduction Percent   0.00%clf_IncomeTaxReconciliationManufacturersDeductionPercent 0.00%clf_IncomeTaxReconciliationManufacturersDeductionPercent (1.60%)clf_IncomeTaxReconciliationManufacturersDeductionPercent (1.60%)clf_IncomeTaxReconciliationManufacturersDeductionPercent 0.90%clf_IncomeTaxReconciliationManufacturersDeductionPercent 0.90%clf_IncomeTaxReconciliationManufacturersDeductionPercent
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount   1,660.6us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance   73.0us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance   634.5us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance  
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent   (17.30%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (17.30%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance 14.90%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance 14.90%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (126.50%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (126.50%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Tax Adjustments, Settlements, and Unusual Provisions   0.2us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions   19.6us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions   (14.8)us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions  
Tax Adjustments Settlements And Unusual Provisions Percent   0.00%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent 0.00%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent 5.30%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent 5.30%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent 2.90%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent 2.90%clf_TaxAdjustmentsSettlementsAndUnusualProvisionsPercent
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent   0.10%us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes (1040000000.00%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes (3.60%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes (1140000000.00%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes 1.10%us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes (570000000.00%)us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount   16.5us-gaap_IncomeTaxReconciliationOtherAdjustments   2.9us-gaap_IncomeTaxReconciliationOtherAdjustments   (1.6)us-gaap_IncomeTaxReconciliationOtherAdjustments  
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent   (0.20%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.20%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.40%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.40%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Income Tax Expense (Benefit) $ (289.7)us-gaap_IncomeTaxExpenseBenefit $ (1,302.0)us-gaap_IncomeTaxExpenseBenefit   $ 55.1us-gaap_IncomeTaxExpenseBenefit   $ 255.9us-gaap_IncomeTaxExpenseBenefit  
Income Tax Expense Benefit Percent   13.60%clf_IncomeTaxExpenseBenefitPercent 13.60%clf_IncomeTaxExpenseBenefitPercent 11.30%clf_IncomeTaxExpenseBenefitPercent 11.30%clf_IncomeTaxExpenseBenefitPercent (51.00%)clf_IncomeTaxExpenseBenefitPercent (51.00%)clf_IncomeTaxExpenseBenefitPercent