Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Benefit Obligations) (Details)

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Benefit Obligations) (Details)
Dec. 31, 2014
Dec. 31, 2013
Pension Benefits [Member] | United States [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 3.83%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
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Expected return on plan assets 8.25%clf_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationReturnOnPlanAssets
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Pension Benefits [Member] | Canada [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
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Expected return on plan assets 7.25%clf_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationReturnOnPlanAssets
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Other Benefits [Member] | United States [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 3.83%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
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Expected return on plan assets 7.00%clf_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationReturnOnPlanAssets
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Other Benefits [Member] | Canada [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Discount rate 3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
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Salary [Member] | Pension Benefits [Member] | United States [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Rate of compensation increase 3.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
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Salary [Member] | Pension Benefits [Member] | Canada [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Rate of compensation increase 3.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
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Hourly [Member] | Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Rate of compensation increase 2.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationRateOfCompensationIncrease
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