Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY Accumulate Other Comprehensive Income (Loss) (Details)

v2.4.1.9
SHAREHOLDERS' EQUITY Accumulate Other Comprehensive Income (Loss) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax $ (251.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax   $ (291.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax $ (204.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax $ (202.0)us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax
Accumulated other comprehensive loss (38.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax   (245.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (112.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (55.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax (0.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax   (1.0)us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax 6.2us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax 10.1us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax 232.4us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax   64.4us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax 106.7us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax 147.2us-gaap_AccumulatedOtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentNetOfTax
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax (18.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax   (18.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax (20.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax (10.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossCumulativeChangesInNetGainLossFromCashFlowHedgesEffectNetOfTax
Accumulated Other Comprehensive Income (Loss) [Member]          
Other Comprehensive Income (Loss), before Reclassifications, before Tax 12.1us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
41.6us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
     
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 195.5clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
16.2clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
     
Accumulated Defined Benefit Plans Adjustment [Member]          
Other Comprehensive Income (Loss), before Reclassifications, before Tax 31.1us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
(0.4)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
     
Accumulated Net Unrealized Investment Gain (Loss) [Member]          
Other Comprehensive Income (Loss), before Reclassifications, before Tax 2.8us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
3.8us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
     
Accumulated Translation Adjustment [Member]          
Other Comprehensive Income (Loss), before Reclassifications, before Tax (14.7)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
40.5us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
     
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]          
Other Comprehensive Income (Loss), before Reclassifications, before Tax (7.1)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
(2.3)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
     
Accumulated Defined Benefit Plans Adjustment [Member]          
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 8.5clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
3.3clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
     
Accumulated Net Unrealized Investment Gain (Loss) [Member]          
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (2.0)clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
0.1clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
     
Accumulated Translation Adjustment [Member]          
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) 182.7clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
0clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedTranslationAdjustmentMember
     
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]          
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) $ 6.3clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
$ 12.8clf_AmountsReclassifiedFromAccumulatedOtherComprehensiveIncomeLoss
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember