XML 71 R86.htm IDEA: XBRL DOCUMENT
v2.4.1.9

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Fair Values of Pension Plan Assets) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Reserve For Investment Commitments | $ 55.5 clf_ReserveForInvestmentCommitments | $ 45.0 clf_ReserveForInvestmentCommitments |  |
Fair value of plan assets | 205.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |  | 188.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets | 200.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 949.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 915.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | 838.7 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Equity Securities [Member] | U S Large Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 248.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |  | 261.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |
Equity Securities [Member] | U S Small Mid Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 55.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |  | 60.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |
Equity Securities [Member] | International [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 157.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_InternationalMember |  | 149.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_InvestmentTypeAxis = clf_InternationalMember |
Fixed Income Investments [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 275.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 244.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Hedge Funds [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 59.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember |  | 57.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember | 85.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember
Hedge Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 59.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 57.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Private Equity Funds [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 31.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember |  | 29.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember | 29.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember
Private Equity Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 31.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 29.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Structured Finance [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 65.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember |  | 61.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember | 56.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember
Redemption Request Notice Period | 90 days |  |  |
Structured Finance [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 65.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 61.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Real Estate [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 50.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember |  | 40.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember | 29.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember
Redemption Request Notice Period | 45 days |  |  |
Real Estate [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 50.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 40.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Cash [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 6.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 10.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 712.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 696.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | U S Large Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 248.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |  | 261.5 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | U S Small Mid Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 55.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |  | 60.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Equity Securities [Member] | International [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 157.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |  | 149.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fixed Income Investments [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 243.7 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 214.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Hedge Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Private Equity Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Structured Finance [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Real Estate [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Cash [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 6.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 10.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Fair Value, Inputs, Level 2 [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 31.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 30.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | U S Large Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | U S Small Mid Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member] | International [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |
Fair Value, Inputs, Level 2 [Member] | Fixed Income Investments [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 31.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 30.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Hedge Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Private Equity Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Structured Finance [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Real Estate [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 [Member] | Cash [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 205.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 188.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | U S Large Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_USLargeCapMember |
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | U S Small Mid Cap [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_USSmallMidCapMember |
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | International [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = clf_InternationalMember |
Significant Unobservable Inputs (Level 3) [Member] | Fixed Income Investments [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeInvestmentsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Hedge Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 59.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 57.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Private Equity Funds [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 31.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 29.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Structured Finance [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 65.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 61.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_StructuredFinanceMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Real Estate [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | 50.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | 40.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Significant Unobservable Inputs (Level 3) [Member] | Cash [Member] | Pension Benefits [Member] |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Fair value of plan assets | $ 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  | $ 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |

X
- Definition Redemption Request Notice Period + References No definition available. + Details
Name: | clf_RedemptionRequestNoticePeriod
Namespace Prefix: | clf_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Reserve for Investment Commitments + References No definition available. + Details
Name: | clf_ReserveForInvestmentCommitments
Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
Name: | us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_CashMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_HedgeFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_RealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_StructuredFinanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |