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Annual report pursuant to Section 13 and 15(d)

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

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Weighted-Average Assumptions Used to Determine Net Benefit Cost) (Details)
 12 Months Ended 2 Months Ended 10 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2012
Oct. 31, 2012
Pension Benefits [Member] | United States [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate 4.57%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		3.70%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		4.28%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember 		Ìý Ìý
Expected return on plan assets 8.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		8.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		8.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		Ìý Ìý
Pension Benefits [Member] | Canada [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate 4.50%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_CanadaMember 		3.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_CanadaMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_PensionPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_CanadaMember 		Ìý Ìý
Expected return on plan assets 7.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_CanadaMember 		7.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_CanadaMember 		Ìý Ìý
Other Benefits [Member] | United States [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate 4.57%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember 		3.70%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember 		Ìý Ìý Ìý
Expected return on plan assets 7.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember 		8.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
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= clf_UnitedStatesMember 		8.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember 		Ìý Ìý
Other Benefits [Member] | Canada [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate 4.75%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_CanadaMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
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= clf_CanadaMember 		4.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_RegionReportingInformationByRegionAxis
= clf_CanadaMember 		Ìý Ìý
Hourly [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Rate of compensation increase 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_HourlyMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_HourlyMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_HourlyMember 		Ìý Ìý
Salary [Member] | Pension Benefits [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Rate of compensation increase 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ clf_TypeofCompensationAxis
= clf_SalaryMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_PensionPlansDefinedBenefitMember
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= clf_SalaryMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_PensionPlansDefinedBenefitMember
/ clf_TypeofCompensationAxis
= clf_SalaryMember 		Ìý Ìý
Salary [Member] | Other Benefits [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Rate of compensation increase 4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_TypeofCompensationAxis
= clf_SalaryMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
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= clf_SalaryMember 		4.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
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= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ clf_TypeofCompensationAxis
= clf_SalaryMember 		Ìý Ìý
Salaried Plan [Member] | United States [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate Ìý Ìý 4.28%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ clf_PlanTypeAxis
= clf_SalariedPlanMember
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= clf_UnitedStatesMember 		Ìý Ìý
Hourly Plan [Member] | United States [Member] Ìý Ìý Ìý Ìý Ìý
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Ìý Ìý Ìý Ìý Ìý
Discount rate Ìý Ìý Ìý 3.51%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
/ clf_PlanTypeAxis
= clf_HourlyPlanMember
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= clf_UnitedStatesMember 		4.28%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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= clf_HourlyPlanMember
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= clf_UnitedStatesMember