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DEBT AND CREDIT FACILITIES (Schedule Of Long-Term Debt) (Details) (USD $) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2014 | Mar. 30, 2015
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | $ 313,700,000 us-gaap_GainsLossesOnExtinguishmentOfDebt |  | $ 0 us-gaap_GainsLossesOnExtinguishmentOfDebt |  |  |  |
Total Face Amount | 3,621,500,000 us-gaap_DebtInstrumentFaceAmount |  |  |  | 3,980,000,000 us-gaap_DebtInstrumentFaceAmount |  |
Letters of credit outstanding | 136,200,000 us-gaap_LettersOfCreditOutstandingAmount |  |  |  | 149,500,000 us-gaap_LettersOfCreditOutstandingAmount |  |
Long-term Debt | 2,880,900,000 us-gaap_LongTermDebt |  |  |  | 2,843,300,000 us-gaap_LongTermDebt |  |
Current portion of long-term debt | 0 us-gaap_LongTermDebtCurrent |  |  |  | 0 us-gaap_LongTermDebtCurrent |  |
Long-term debt noncurrent portion | 2,880,900,000 us-gaap_LongTermDebtNoncurrent |  |  |  | 2,843,300,000 us-gaap_LongTermDebtNoncurrent |  |
$700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 58,300,000 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 Repurchase Discount | 52.00%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 208,500,000 clf_DebtInstrumentFaceAmountExchanged / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  |  |  |
Type | Fixed |  |  | Fixed |  |  |
Final Maturity | 2021 |  |  | 2021 |  |  |
Total Face Amount | 700,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  | 700,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Debt Instrument, Par Value | 423,200,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  | 690,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Long-term Debt | 422,900,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember | [1] |  |  | 689,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember | [1] |
Debt Instrument, Unamortized Discount | 300,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  | 500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
Imputed interest rate | 4.89%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |  |  | 4.88%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember |  |
$500 million 4.80% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 43,800,000 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 |  |
Debt Repurchase Discount | 54.30%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 137,800,000 clf_DebtInstrumentFaceAmountExchanged / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  |  |  |
Type | Fixed |  |  | Fixed |  |  |
Final Maturity | 2020 |  |  | 2020 |  |  |
Total Face Amount | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Debt Instrument, Par Value | 308,500,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  | 490,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Long-term Debt | 308,100,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember | [2] |  |  | 489,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember | [2] |
Debt Instrument, Unamortized Discount | 400,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  | 600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
Imputed interest rate | 4.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |  |  | 4.83%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember |  |
$800 Million 6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 45,900,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |  |  |
Stated interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |  |  |
Debt Repurchase Discount | 52.50%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 261,300,000 clf_DebtInstrumentFaceAmountExchanged / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |  |  |
Type | Fixed |  |  | Fixed |  |  |
Final Maturity | 2040 |  |  | 2040 |  |  |
Total Face Amount | 800,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  |  |  |
Debt Instrument, Par Value | 492,800,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  | 800,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Long-term Debt | 487,000,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember | [3] |  |  | 790,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember | [3] |
Debt Instrument, Unamortized Discount | 5,800,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  | 9,500,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
Imputed interest rate | 6.34%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |  |  | 6.34%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember |  |
$400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 1,300,000 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 |  |
Debt Repurchase Discount | 58.00%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 67,000,000 clf_DebtInstrumentFaceAmountExchanged / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  |  |  |
Type | Fixed |  |  | Fixed |  |  |
Final Maturity | 2020 |  |  | 2020 |  |  |
Total Face Amount | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  | 400,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Debt Instrument, Par Value | 326,800,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  | 395,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Long-term Debt | 325,700,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember | [4] |  |  | 393,700,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember | [4] |
Debt Instrument, Unamortized Discount | 1,100,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  | 1,300,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
Imputed interest rate | 5.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |  |  | 5.98%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember |  |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Repurchased Face Amount | 44,000,000 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 Repurchase Discount | 77.50%clf_DebtRepurchaseDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  |  |  |
Type | Fixed |  |  | Fixed |  |  |
Final Maturity | 2018 |  |  | 2018 |  |  |
Total Face Amount | 500,000,000.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Debt Instrument, Par Value | 436,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  | 480,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Long-term Debt | 433,800,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | [5] |  |  | 477,400,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | [5] |
Debt Instrument, Unamortized Discount | 2,200,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  | 2,600,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Imputed interest rate | 6.32%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |  |  | 5.17%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Five hundred forty million Eight point two five Twenty twenty First Lien Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Stated interest rate | 8.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |  |  |  |  |
Type | Fixed |  |  |  |  |  |
Final Maturity | 2020 |  |  |  |  |  |
Total Face Amount | 540,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |  |  | 0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |
Debt Instrument, Par Value | 540,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |  |  |  |  |
Long-term Debt | 503,500,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember | [5] |  |  |  |  |
Debt Instrument, Unamortized Discount | 36,500,000.0 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |  |  |  |  |
Imputed interest rate | 9.97%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember |  |  |  |  |  |
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Stated interest rate | 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Discount Received in Debt Exchange of $400M 5.90% Notes | 15,500,000 clf_DiscountReceivedinDebtExchangeof400M5.90Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Debt Instrument, Face Amount Received in Debt Exchange of $400M 5.90% Notes | 57,500,000 clf_DebtInstrumentFaceAmountReceivedinDebtExchangeof400M5.90Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Debt Instrument, Face Amount Received in Debt Exchange of $800M 6.25% Notes | 203,500,000 clf_DebtInstrumentFaceAmountReceivedinDebtExchangeof800M6.25Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Discount Received in Debt Exchange of $800M 6.25% Notes | 55,000,000 clf_DiscountReceivedinDebtExchangeof800M6.25Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Discount Received in Debt Exchange of $500M 4.80% Notes | 30,500,000 clf_DiscountReceivedinDebtExchangeof500M4.80Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Debt Instrument, Face Amount Received in Debt Exchange of $500M 4.80% Notes | 112,900,000 clf_DebtInstrumentFaceAmountReceivedinDebtExchangeof500M4.80Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Discount Received in Debt Exchange of $700M 4.875% Notes | 46,000,000 clf_DiscountReceivedinDebtExchangeof700M4.875Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Debt instrument, Face Amount Received in Debt Exchange of $700M 4.875% Notes | 170,300,000 clf_DebtinstrumentFaceAmountReceivedinDebtExchangeof700M4.875Notes / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Type | Fixed |  |  |  |  |  |
Final Maturity | 2020 |  |  |  |  |  |
Total Face Amount | 544,200,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  | 0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |
Debt Instrument, Par Value | 544,200,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Long-term Debt | 397,200,000 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember | [5] |  |  |  |  |
Debt Instrument, Unamortized Discount | 147,000,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  |  |  |
Imputed interest rate | 15.55%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |  |  | 15.55%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Type | Variable |  |  | Variable |  |  |
Final Maturity | 2020 |  |  | 2017 |  |  |
Debt Instrument, Par Value | 550,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  | 1,125,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  | 900,000,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember
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 | [7] |
Revolving credit facility, borrowing capacity | 441,100,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |
Letters of credit outstanding | 136,200,000 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  | 149,500,000 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Foreign Exchange Hedge Obligations | 5,500,000 clf_ForeignExchangeHedgeObligations / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |
Imputed interest rate | 0.00%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  | 2.94%us-gaap_DebtInstrumentInterestRateEffectivePercentage / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Credit facility remaining capacity | 299,400,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  | 975,500,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_RevolvingCreditFacilityMember |  |
Interest Rate Swap [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Fair Value Adjustment to Interest Rate Hedge | 2,700,000 clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis = us-gaap_InterestRateSwapMember |  |  |  | 2,800,000 clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis = us-gaap_InterestRateSwapMember |  |
Debt Repurchase [Member] | $700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 20,000,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_DebtRepurchaseMember |  |  |  |  |  |
Debt Repurchase [Member] | $500 million 4.80% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 15,600,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_DebtRepurchaseMember |  |  |  |  |  |
Debt Repurchase [Member] | $800 Million 6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 15,000,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember / us-gaap_ExtinguishmentOfDebtAxis = clf_DebtRepurchaseMember |  |  |  |  |  |
Debt Repurchase [Member] | $400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 300,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_DebtRepurchaseMember |  |  |  |  |  |
Debt Repurchase [Member] | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 7,100,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_DebtRepurchaseMember |  |  |  |  |  |
Exchange of Debt [Member] | $700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 83,100,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_ExchangeofDebtMember |  |  |  |  |  |
Exchange of Debt [Member] | $500 million 4.80% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 54,600,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_ExchangeofDebtMember |  |  |  |  |  |
Exchange of Debt [Member] | $800 Million 6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | 107,300,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember / us-gaap_ExtinguishmentOfDebtAxis = clf_ExchangeofDebtMember |  |  |  |  |  |
Exchange of Debt [Member] | $400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Gain on extinguishment of debt | $ 24,500,000 us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_DebtInstrumentAxis = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember / us-gaap_ExtinguishmentOfDebtAxis = clf_ExchangeofDebtMember |  |  |  |  |  |
[1] | During the first quarter of 2015, we purchased $58.3 million of outstanding 4.875 percent senior notes that were trading at a discount of 52.0 percent which resulted in a gain on extinguishment of $20.0 million. In addition, on March 27, 2015, we exchanged as part of a tender offer $208.5 million of the 4.875 percent senior notes for $170.3 million of the 7.75 percent second lien notes at a discount of $46.0 million based on an imputed interest rate of 15.55 percent, resulting in a gain on extinguishment of $83.1 million, net of amounts expensed for unamortized original issue discount and deferred origination fees. As of March 31, 2015, the $700.0 million 4.875 percent senior notes were recorded at a par value of $423.2 million less unamortized discounts of $0.3 million, based on an imputed interest rate of 4.89 percent. 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.
[2] | During the first quarter of 2015, we purchased $43.8 million of outstanding 4.80 percent senior notes that were trading at a discount of 54.3 percent, which resulted in a gain on extinguishment of $15.6 million. In addition, on March 27, 2015, we exchanged as part of a tender offer $137.8 million of the 4.80 percent senior notes for $112.9 million of the 7.75 percent second lien notes at a discount of $30.5 million based on an imputed interest rate of 15.55 percent, resulting in a gain on extinguishment of $54.6 million, net of amounts expensed for unamortized original issue discount and deferred origination fees. As of March 31, 2015, the $500.0 million 4.80 percent senior notes were recorded at a par value of $308.5 million less unamortized discounts of $0.4 million, based on an imputed interest rate of 4.83 percent. 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.
[3] | During the first quarter of 2015, we purchased $45.9 million of outstanding 6.25 percent senior notes that were trading at a discount of 52.5 percent, which resulted in a gain on extinguishment of $15.0 million. In addition, on March 27, 2015, we exchanged as part of a tender offer $261.3 million of the 6.25 percent senior notes for $203.5 million of the 7.75 percent second lien notes at a discount of $55.0 million based on an imputed interest rate of 15.55 percent, resulting in a gain on extinguishment of $107.3 million, net of amounts expensed for unamortized original issue discount and deferred origination fees. As of March 31, 2015, the $800 million 6.25 percent senior notes were recorded at a par value of $492.8 million less unamortized discounts of $5.8 million, based on an imputed interest rate of 6.34 percent. As of December 31, 2014, the $800 million 6.25 percent senior notes were recorded at a par value of $800.0 million less unamortized discounts of $9.5 million based on an imputed interest rate of 6.34 percent.
[4] | During the first quarter of 2015, we purchased $1.3 million of outstanding 5.90 percent senior notes that were trading at a discount of 58.0 percent, which resulted in a gain on extinguishment of $0.3 million. In addition, on March 27, 2015, we exchanged as part of a tender offer $67.0 million of the 5.90 percent senior notes for $57.5 million of the 7.75 percent second lien notes at a discount of $15.5 million based on an imputed interest rate of 15.55 percent, resulting in a gain on extinguishment of $24.5 million, net of amounts expensed for unamortized original issue discount and deferred origination fees. As of March 31, 2015, the $400.0 million 5.90 percent senior notes were recorded at a par value of $326.8 million less unamortized discounts of $1.1 million, based on an imputed interest rate of 5.98 percent. 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.
[5] | During the first quarter of 2015, we purchased $44.0 million of outstanding 3.95 percent senior notes that were trading at a discount of 77.5 percent, which resulted in a gain on the extinguishment of debt of $7.1 million. As of March 31, 2015, the $500.0 million 3.95 percent senior notes were recorded at a par value of $436.0 million less unamortized discounts of $2.2 million, based on an imputed interest rate of 6.32 percent. 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.
[6] | As of March 31, 2015, no loans were drawn under the ABL Facility and we had total availability of $441.1 million as a result of borrowing base limitations. As of March 31, 2015, the principal amount of letter of credit obligations totaled $136.2 million and foreign exchange hedge obligations totaled $5.5 million, thereby further reducing available borrowing capacity on our ABL Facility to $299.4 million.
[7] | As of December 31, 2014, we had no revolving loans drawn under the revolving credit agreement of which had $1.125 billion availability. As of December 31, 2014, the principal amount of letter of credit obligations totaled $149.5 million, thereby further reducing available borrowing capacity on the revolving credit agreement to $975.5 million.

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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