Annual report pursuant to Section 13 and 15(d)

INCOME TAXES INCOME TAXES (Schedule of Components of Income Taxes) (Details)

v2.4.1.9
INCOME TAXES INCOME TAXES (Schedule of Components of Income Taxes) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Contingency [Line Items]        
Current Federal Tax Expense (Benefit)   $ (159.9)us-gaap_CurrentFederalTaxExpenseBenefit $ 101.3us-gaap_CurrentFederalTaxExpenseBenefit $ 71.1us-gaap_CurrentFederalTaxExpenseBenefit
Current State and Local Tax Expense (Benefit)   (0.6)us-gaap_CurrentStateAndLocalTaxExpenseBenefit 4.0us-gaap_CurrentStateAndLocalTaxExpenseBenefit 7.6us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Current Foreign Tax Expense (Benefit)   17.2us-gaap_CurrentForeignTaxExpenseBenefit 87.9us-gaap_CurrentForeignTaxExpenseBenefit 50.2us-gaap_CurrentForeignTaxExpenseBenefit
Current Income Tax Expense (Benefit)   (143.3)us-gaap_CurrentIncomeTaxExpenseBenefit 193.2us-gaap_CurrentIncomeTaxExpenseBenefit 128.9us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred Federal Income Tax Expense (Benefit)   (258.9)us-gaap_DeferredFederalIncomeTaxExpenseBenefit 23.3us-gaap_DeferredFederalIncomeTaxExpenseBenefit 221.2us-gaap_DeferredFederalIncomeTaxExpenseBenefit
Deferred State and Local Income Tax Expense (Benefit)   (43.0)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 3.0us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 1.4us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Deferred Foreign Income Tax Expense (Benefit)   (856.8)us-gaap_DeferredForeignIncomeTaxExpenseBenefit (164.4)us-gaap_DeferredForeignIncomeTaxExpenseBenefit (95.6)us-gaap_DeferredForeignIncomeTaxExpenseBenefit
Deferred Income Tax Expense (Benefit)   (1,153.9)us-gaap_DeferredIncomeTaxExpenseBenefit (138.1)us-gaap_DeferredIncomeTaxExpenseBenefit 127.0us-gaap_DeferredIncomeTaxExpenseBenefit
Income Tax Expense (Benefit) (289.7)us-gaap_IncomeTaxExpenseBenefit (1,302.0)us-gaap_IncomeTaxExpenseBenefit 55.1us-gaap_IncomeTaxExpenseBenefit 255.9us-gaap_IncomeTaxExpenseBenefit
Deferred Provision (Benefit) [Member]        
Income Tax Contingency [Line Items]        
Deferred Income Tax Expense (Benefit)   $ (1,158.7)us-gaap_DeferredIncomeTaxExpenseBenefit
/ clf_IncomeTaxDescriptionAxis
= clf_DeferredProvisionBenefitMember 		$ (138.1)us-gaap_DeferredIncomeTaxExpenseBenefit
/ clf_IncomeTaxDescriptionAxis
= clf_DeferredProvisionBenefitMember 		$ 127.0us-gaap_DeferredIncomeTaxExpenseBenefit
/ clf_IncomeTaxDescriptionAxis
= clf_DeferredProvisionBenefitMember