Annual report pursuant to Section 13 and 15(d)

DEBT AND CREDIT FACILITIES (Schedule Of Long-Term Debt) (Details)

v2.4.1.9
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 $ 1,000,000,000clf_MaximumImpairmentPermittedtobeExcludedfromNetWorthCalculationforDebtCovenants $ 1,000,000,000clf_MaximumImpairmentPermittedtobeExcludedfromNetWorthCalculationforDebtCovenants    
Stock Repurchase Program, Authorized Amount 200,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1 200,000,000us-gaap_StockRepurchaseProgramAuthorizedAmount1    
Debt Instrument, Par Value 4,144,800,000clf_DebtInstrumentParValue 4,144,800,000clf_DebtInstrumentParValue 4,814,800,000clf_DebtInstrumentParValue  
Long-term Debt 2,984,100,000us-gaap_LongTermDebt 2,984,100,000us-gaap_LongTermDebt 3,043,500,000us-gaap_LongTermDebt  
Current portion of long-term debt 21,800,000us-gaap_LongTermDebtCurrent 21,800,000us-gaap_LongTermDebtCurrent 20,900,000us-gaap_LongTermDebtCurrent  
Long-term debt noncurrent portion 2,962,300,000us-gaap_LongTermDebtNoncurrent 2,962,300,000us-gaap_LongTermDebtNoncurrent 3,022,600,000us-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 10,000,000us-gaap_DebtInstrumentRepurchasedFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
10,000,000us-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 690,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
690,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
700,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 
Debt Instrument, Unamortized Discount 500,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
500,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
600,000us-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   4,100,000us-gaap_GainsLossesOnExtinguishmentOfDebt
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
   
Type   Fixed Fixed  
Final Maturity   2021 2021  
Long-term Debt 689,500,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
[1] 689,500,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
[1] 699,400,000us-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 700,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
700,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
700,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 
$500 million 4.80% 2020 Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Repurchased Face Amount 10,000,000us-gaap_DebtInstrumentRepurchasedFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
10,000,000us-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 490,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
490,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
500,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
 
Debt Instrument, Unamortized Discount 600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
800,000us-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   4,000,000us-gaap_GainsLossesOnExtinguishmentOfDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
   
Type   Fixed Fixed  
Final Maturity   2020 2020  
Long-term Debt 489,400,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
[2] 489,400,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
[2] 499,200,000us-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 500,000,000.0us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
500,000,000.0us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionFourPointEightZeroPercentSeniorNoteMember
500,000,000us-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 800,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
800,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
800,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
 
Debt Instrument, Unamortized Discount 9,500,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
9,500,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
9,600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
 
Type   Fixed Fixed  
Final Maturity   2040 2040  
Long-term Debt 790,500,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
[3] 790,500,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
[3] 790,400,000us-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 800,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
800,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_EightHundredMillionSixPointTwoFivePercentSeniorNotesDueTwoThousandFortyMember
   
$400 Million 5.90% 2020 Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Repurchased Face Amount 5,000,000us-gaap_DebtInstrumentRepurchasedFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
5,000,000us-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 395,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
395,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
400,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 
Debt Instrument, Unamortized Discount 1,300,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
1,300,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
1,600,000us-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   1,900,000.0us-gaap_GainsLossesOnExtinguishmentOfDebt
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
   
Type   Fixed Fixed  
Final Maturity   2020 2020  
Long-term Debt 393,700,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
[4] 393,700,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
[4] 398,400,000us-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 400,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
400,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
400,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 
$500 Million 3.95% 2018 Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Repurchased Face Amount 20,000,000us-gaap_DebtInstrumentRepurchasedFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
20,000,000us-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 480,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
480,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
500,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 
Debt Instrument, Unamortized Discount 2,600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
2,600,000us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
3,500,000us-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   6,200,000us-gaap_GainsLossesOnExtinguishmentOfDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
   
Type   Fixed Fixed  
Final Maturity   2018 2018  
Long-term Debt 477,400,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
[5] 477,400,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
[5] 496,500,000us-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 500,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
500,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
500,000,000.0us-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 3.5clf_CoverageRatioOfEarningsBeforeInterestTaxDepreciationAndAmortizationToTotalInterestExpense
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
  2.5clf_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 2.0clf_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 60clf_NumberofDaystocompletecollateralactioninordertoreduceminimuminterestcoverageratioSubsequentEventRevolvingCreditFacilityAmendment
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
     
Debt Instrument, Par Value 1,125,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
1,125,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
1,750,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
1,250,000,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
Type   Variable Variable  
Final Maturity   2017 2017  
Credit facility, amount outstanding 0us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
[6] 0us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
[6] 0us-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 1,000,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
1,000,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
1,700,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_RevolvingCreditFacilityMember
 
Letter of Credit [Member]        
Debt Instrument [Line Items]        
Credit facility, amount outstanding 149,500,000us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LetterOfCreditMember
149,500,000us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LetterOfCreditMember
8,400,000us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LetterOfCreditMember
 
Equipment Loans [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Par Value 164,800,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_EquipmentLoansMember
164,800,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_EquipmentLoansMember
   
Type   Fixed Fixed  
Long-term Debt 140,800,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_EquipmentLoansMember
140,800,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_EquipmentLoansMember
161,700,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= clf_EquipmentLoansMember
 
Interest Rate Swap [Member]        
Debt Instrument [Line Items]        
Fair Value Adjustment to Interest Rate Hedge 2,800,000clf_FairValueAdjustmenttoInterestRateHedge
/ us-gaap_DebtInstrumentAxis
= us-gaap_InterestRateSwapMember
2,800,000clf_FairValueAdjustmenttoInterestRateHedge
/ us-gaap_DebtInstrumentAxis
= us-gaap_InterestRateSwapMember
(2,100,000)clf_FairValueAdjustmenttoInterestRateHedge
/ us-gaap_DebtInstrumentAxis
= us-gaap_InterestRateSwapMember
 
Senior Notes Five Hundred Million [Member]        
Debt Instrument [Line Items]        
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 0.50%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis
/ us-gaap_DebtInstrumentAxis
= clf_SeniorNotesFiveHundredMillionMember
0.50%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis
/ us-gaap_DebtInstrumentAxis
= clf_SeniorNotesFiveHundredMillionMember
   
Senior Notes10 Year Tranche [Member]        
Debt Instrument [Line Items]        
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 0.25%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 300,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember
300,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember
   
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_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember
0.40%clf_DiscountRateUsedToCalculatePresentValuesOfRemainingScheduledPaymentsOfPrincipalAndInterestOnNotesToBeRedeemedSpreadOnRateBasis
/ us-gaap_DebtInstrumentAxis
= clf_ThreeHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember
   
Five Hundred Million Six Point Two Five Percent Two Thousand And Forty [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Face Amount $ 500,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= clf_FiveHundredMillionSixPointTwoFivePercentTwoThousandAndFortyMember
$ 500,000,000us-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.