Annual report pursuant to Section 13 and 15(d)

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) Accumulated Other Comprehensive Income (Loss) and Related Tax Effects (Details)

v2.4.1.9
ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) Accumulated Other Comprehensive Income (Loss) and Related Tax Effects (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss), Before Taxes $ (388.1)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes       $ (213.3)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes       $ (244.7)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes  
Accumulated Other Comprehensive Income (Loss), Tax 142.3clf_AccumulatedOtherComprehensiveIncomeLossTax       100.4clf_AccumulatedOtherComprehensiveIncomeLossTax       189.1clf_AccumulatedOtherComprehensiveIncomeLossTax  
Accumulated other comprehensive loss (245.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (101.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (20.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (55.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (112.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (178.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (239.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (51.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (55.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (92.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Postretirement Benefit Liability [Member]                    
Accumulated Other Comprehensive Income (Loss), Before Taxes (425.3)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      (299.3)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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= clf_PostretirementBenefitLiabilityMember
      (576.7)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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Accumulated Other Comprehensive Income (Loss), Tax 134.2clf_AccumulatedOtherComprehensiveIncomeLossTax
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      94.4clf_AccumulatedOtherComprehensiveIncomeLossTax
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      194.0clf_AccumulatedOtherComprehensiveIncomeLossTax
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Accumulated other comprehensive loss (291.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      (204.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      (382.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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Foreign Currency Translation Adjustment [Member]                    
Accumulated Other Comprehensive Income (Loss), Before Taxes 64.4clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      106.7clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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= clf_ForeignCurrencyTranslationAdjustmentMember
      316.3clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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Accumulated Other Comprehensive Income (Loss), Tax 0clf_AccumulatedOtherComprehensiveIncomeLossTax
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      0clf_AccumulatedOtherComprehensiveIncomeLossTax
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      0clf_AccumulatedOtherComprehensiveIncomeLossTax
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Accumulated other comprehensive loss 64.4us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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= clf_ForeignCurrencyTranslationAdjustmentMember
      106.7us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      316.3us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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Unrealized Gain Loss On Derivatives [Member]                    
Accumulated Other Comprehensive Income (Loss), Before Taxes (25.9)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      (30.0)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      12.4clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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Accumulated Other Comprehensive Income (Loss), Tax 7.8clf_AccumulatedOtherComprehensiveIncomeLossTax
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= clf_UnrealizedGainLossOnDerivativesMember
      9.1clf_AccumulatedOtherComprehensiveIncomeLossTax
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= clf_UnrealizedGainLossOnDerivativesMember
      (3.7)clf_AccumulatedOtherComprehensiveIncomeLossTax
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Accumulated other comprehensive loss (18.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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= clf_UnrealizedGainLossOnDerivativesMember
      (20.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      8.7us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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Accumulated Net Unrealized Investment Gain (Loss) [Member]                    
Accumulated Other Comprehensive Income (Loss), Before Taxes (1.3)clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      9.3clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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      3.3clf_AccumulatedOtherComprehensiveIncomeLossBeforeTaxes
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Accumulated Other Comprehensive Income (Loss), Tax 0.3clf_AccumulatedOtherComprehensiveIncomeLossTax
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      (3.1)clf_AccumulatedOtherComprehensiveIncomeLossTax
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      (1.2)clf_AccumulatedOtherComprehensiveIncomeLossTax
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Accumulated other comprehensive loss $ (1.0)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      $ 6.2us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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      $ 2.1us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
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