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DEBT AND CREDIT FACILITIES (Schedule Of Long-Term Debt) (Details) (USD $) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2014
Debt Instrument [Line Items] |  |  |  |  |  |  |
Maximum Impairment Permitted to be Excluded from Net Worth Calculation for Debt Covenants | $ 1,000,000,000 clf_MaximumImpairmentPermittedtobeExcludedfromNetWorthCalculationforDebtCovenants |  | $ 1,000,000,000 clf_MaximumImpairmentPermittedtobeExcludedfromNetWorthCalculationforDebtCovenants |  |  |  |
Stock Repurchase Program, Authorized Amount | 200,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount1 |  | 200,000,000 us-gaap_StockRepurchaseProgramAuthorizedAmount1 |  |  |  |
Debt Instrument, Par Value | 4,144,800,000 clf_DebtInstrumentParValue |  | 4,144,800,000 clf_DebtInstrumentParValue |  | 4,814,800,000 clf_DebtInstrumentParValue |  |
Long-term Debt | 2,984,100,000 us-gaap_LongTermDebt |  | 2,984,100,000 us-gaap_LongTermDebt |  | 3,043,500,000 us-gaap_LongTermDebt |  |
Current portion of long-term debt | 21,800,000 us-gaap_LongTermDebtCurrent |  | 21,800,000 us-gaap_LongTermDebtCurrent |  | 20,900,000 us-gaap_LongTermDebtCurrent |  |
Long-term debt noncurrent portion | 2,962,300,000 us-gaap_LongTermDebtNoncurrent |  | 2,962,300,000 us-gaap_LongTermDebtNoncurrent |  | 3,022,600,000 us-gaap_LongTermDebtNoncurrent |  |
Seven Hundred Million Four Point Eight Seven Five Two Thousand Twenty-one Senior Note [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 10,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 10,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |
Stated interest rate | 4.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 4.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |
Debt Instrument, Par Value | 690,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 690,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 700,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Debt Instrument, Unamortized Discount | 500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Debt Repurchase Discount | 40.50%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 40.50%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |
Gains (Losses) on Extinguishment of Debt |  |  | 4,100,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |
Type |  |  | Fixed |  | Fixed |  |
Final Maturity |  |  | 2021 |  | 2021 |  |
Long-term Debt | 689,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember | [1] | 689,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember | [1] | 699,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember | [1] |
Imputed interest rate | 4.88%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 4.88%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 4.88%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Debt Instrument, Face Amount | 700,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 700,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  | 700,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
$500 million 4.80% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 10,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 10,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |
Stated interest rate | 4.80%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 4.80%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 4.80%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Debt Instrument, Par Value | 490,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 490,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 500,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Debt Instrument, Unamortized Discount | 600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 800,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Debt Repurchase Discount | 40.25%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 40.25%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |
Gains (Losses) on Extinguishment of Debt |  |  | 4,000,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |
Type |  |  | Fixed |  | Fixed |  |
Final Maturity |  |  | 2020 |  | 2020 |  |
Long-term Debt | 489,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember | [2] | 489,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember | [2] | 499,200,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember | [2] |
Imputed interest rate | 4.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 4.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 4.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Debt Instrument, Face Amount | 500,000,000.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 500,000,000.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Discount Rate Used to Calculate Present Values of Remaining Scheduled Payments of Principal and Interest on Notes to Be Redeemed, Variable Rate Basis |  |  | treasury rate |  |  |  |
Discount Rate Used to Calculate Present Values of Remaining Scheduled Payments of Principal and Interest on Notes to Be Redeemed, Spread on Rate Basis | 0.35%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  | 0.35%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |
$800 Million 6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Stated interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Debt Instrument, Par Value | 800,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 800,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 800,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Debt Instrument, Unamortized Discount | 9,500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 9,500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 9,600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Type |  |  | Fixed |  | Fixed |  |
Final Maturity |  |  | 2040 |  | 2040 |  |
Long-term Debt | 790,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember | [3] | 790,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember | [3] | 790,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember | [3] |
Imputed interest rate | 6.34%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 6.34%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 6.34%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Debt Instrument, Face Amount | 800,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  | 800,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |
$400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 5,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 5,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  |
Stated interest rate | 5.90%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 5.90%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 5.90%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Debt Instrument, Par Value | 395,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 395,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 400,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Debt Instrument, Unamortized Discount | 1,300,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 1,300,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 1,600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Debt Repurchase Discount | 38.125%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 38.125%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  |
Gains (Losses) on Extinguishment of Debt |  |  | 1,900,000.0 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  |
Type |  |  | Fixed |  | Fixed |  |
Final Maturity |  |  | 2020 |  | 2020 |  |
Long-term Debt | 393,700,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember | [4] | 393,700,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember | [4] | 398,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember | [4] |
Imputed interest rate | 5.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 5.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 5.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Debt Instrument, Face Amount | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 20,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 20,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  |
Stated interest rate | 3.95%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 3.95%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  |
Debt Instrument, Par Value | 480,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 480,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 500,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Debt Instrument, Unamortized Discount | 2,600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 2,600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 3,500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Debt Repurchase Discount | 30.875%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 30.875%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  |
Gains (Losses) on Extinguishment of Debt |  |  | 6,200,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  |
Type |  |  | Fixed |  | Fixed |  |
Final Maturity |  |  | 2018 |  | 2018 |  |
Long-term Debt | 477,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | [5] | 477,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | [5] | 496,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | [5] |
Imputed interest rate | 5.17%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 5.17%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 4.14%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Debt Instrument, Face Amount | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  | 500,000,000.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Coverage Ratio Of Earnings Before Interest Tax Depreciation And Amortization To Total Interest Expense | 3.5 clf_CoverageRatioOfEarningsBeforeInterestTaxDepreciationAndAmortizationToTotalInterestExpense / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  | 2.5 clf_CoverageRatioOfEarningsBeforeInterestTaxDepreciationAndAmortizationToTotalInterestExpense / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Coverage Ratio Of Earnings Before Interest Tax Depreciation And Amortization To Total Interest Expense, Subsequent Event - Revolving Credit Facility Amendment | 2.0 clf_CoverageRatioOfEarningsBeforeInterestTaxDepreciationAndAmortizationToTotalInterestExpenseSubsequentEventRevolvingCreditFacilityAmendment / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |
Number of Days to complete collateral action in order to reduce minimum interest coverage ratio, Subsequent Event - Revolving Credit Facility Amendment | 60 clf_NumberofDaystocompletecollateralactioninordertoreduceminimuminterestcoverageratioSubsequentEventRevolvingCreditFacilityAmendment / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |
Debt Instrument, Par Value | 1,125,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1,125,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1,750,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1,250,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember
Type |  |  | Variable |  | Variable |  |
Final Maturity |  |  | 2017 |  | 2017 |  |
Credit facility, amount outstanding | 0 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember | [6] | 0 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember | [6] | 0 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember | [6] |
Imputed interest rate | 2.94%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 2.94%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1.64%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Credit facility remaining capacity | 1,000,000,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1,000,000,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 1,700,000,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Letter of Credit [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, amount outstanding | 149,500,000 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LetterOfCreditMember |  | 149,500,000 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LetterOfCreditMember |  | 8,400,000 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LetterOfCreditMember |  |
Equipment Loans [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Par Value | 164,800,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EquipmentLoansMember |  | 164,800,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EquipmentLoansMember |  |  |  |
Type |  |  | Fixed |  | Fixed |  |
Long-term Debt | 140,800,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EquipmentLoansMember |  | 140,800,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EquipmentLoansMember |  | 161,700,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EquipmentLoansMember |  |
Interest Rate Swap [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
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Debt Instrument [Line Items] |  |  |  |  |  |  |
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Debt Instrument [Line Items] |  |  |  |  |  |  |
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Discount Rate Used to Calculate Present Values of Remaining Scheduled Payments of Principal and Interest on Notes to Be Redeemed, Spread on Rate Basis | 0.25%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_SeniorNotes10YearTrancheMember |  | 0.25%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_SeniorNotes10YearTrancheMember |  |  |  |
Senior Notes30 Year Tranche [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Face Amount | 300,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  | 300,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  |  |  |
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Five Hundred Million Six Point Two Five Percent Two Thousand And Forty [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Face Amount | $ 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  | $ 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  |  |  |
Discount Rate Used to Calculate Present Values of Remaining Scheduled Payments of Principal and Interest on Notes to Be Redeemed, Variable Rate Basis |  |  | treasury rate |  |  |  |
Discount Rate Used to Calculate Present Values of Remaining Scheduled Payments of Principal and Interest on Notes to Be Redeemed, Spread on Rate Basis | 0.40%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  | 0.40%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember |  |  |  |
[1] | During the fourth quarter of 2014, we purchased $10.0 million of outstanding 4.875 percent senior notes that were trading at a discount of 40.5 percent which resulted in a gain on the extinguishment of debt of $4.1 million. As of December 31, 2014, the $700.0 million 4.875 percent senior notes were recorded at a par value of $690.0 million less unamortized discounts of $0.5 million, based on an imputed interest rate of 4.88 percent. As of December 31, 2013, the $700.0 million 4.875 percent senior notes were recorded at a par value of $700.0 million less unamortized discounts of $0.6 million, based on an imputed interest rate of 4.88 percent.
[2] | During the fourth quarter of 2014, we purchased $10.0 million of outstanding 4.80 percent senior notes that were trading at a discount of 40.25 percent which resulted in a gain on the extinguishment of debt of $4.0 million. As of December 31, 2014, the $500.0 million 4.80 percent senior notes were recorded at a par value of $490.0 million less unamortized discounts of $0.6 million, based on an imputed interest rate of 4.83 percent. As of December 31, 2013, the $500.0 million 4.80 percent senior notes were recorded at a par value of $500.0 million less unamortized discounts of $0.8 million, based on an imputed interest rate of 4.83 percent.
[3] | As of December 31, 2014 and December 31, 2013, the $800.0 million 6.25 percent senior notes were recorded at par value of $800.0 million less unamortized discounts of $9.5 million and $9.6 million, respectively, based on an imputed interest rate of 6.34 percent.
[4] | During the fourth quarter of 2014, we purchased $5.0 million of outstanding 5.90 percent senior notes that were trading at a discount of 38.125 percent which resulted in a gain on the extinguishment of debt of $1.9 million. As of December 31, 2014, the $400.0 million 5.90 percent senior notes were recorded at a par value of $395.0 million less unamortized discounts of $1.3 million, based on an imputed interest rate of 5.98 percent. As of December 31, 2013, the $400.0 million 5.90 percent senior notes were recorded at a par value of $400.0 million less unamortized discounts of $1.6 million, based on an imputed interest rate of 5.98 percent.
[5] | During the fourth quarter of 2014, we purchased $20.0 million of outstanding 3.95 percent senior notes that were trading at a discount of 30.875 percent which resulted in a gain on the extinguishment of debt of $6.2 million. As of December 31, 2014, the $500.0 million 3.95 percent senior notes were recorded at a par value of $480.0 million less unamortized discounts of $2.6 million, based on an imputed interest rate of 5.17 percent. As of December 31, 2013, the $500.0 million 3.95 percent senior notes were recorded at a par value of $500.0 million less unamortized discounts of $3.5 million, based on an imputed interest rate of 4.14 percent.
[6] | As of December 31, 2014 and 2013, no revolving loans were drawn under the credit facility. We had total availability of $1.125 billion and $1.75 billion on our credit facility as of December 31, 2014 and 2013, respectively. Additionally, as of December 31, 2014 and December 31, 2013, the principal amount of letter of credit obligations totaled $149.5 million and $8.4 million, respectively, thereby reducing available borrowing capacity to $1.0 billion and $1.7 billion for each period, respectively.

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