Annual report pursuant to Section 13 and 15(d)

DEBT AND CREDIT FACILITIES DEBT AND CREDIT FACILITIES (Schedule of Debt Maturities) (Details)

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DEBT AND CREDIT FACILITIES DEBT AND CREDIT FACILITIES (Schedule of Debt Maturities) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Debt Disclosure [Abstract]  
Debt Maturities 2015 $ 21.8us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths
Debt Maturities 2016 22.7us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
Debt Maturities 2017 23.6us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree
Debt Maturities 2018 504.6us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour
Debt Maturities 2019 25.5us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive
Debt Maturities 2020 and After 2,397.6us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive
Long-term Debt, Maturities, Total $ 2,995.8clf_LongTermDebtMaturitiesTotal