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PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Assumed Health Care Cost Trend Rates) (Details) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Ultimate health care cost trend rate | 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate | 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
United States [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Health care cost trend rate assumed for next year | 7.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear / clf_RegionReportingInformationByRegionAxis = clf_UnitedStatesMember | 7.25%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear / clf_RegionReportingInformationByRegionAxis = clf_UnitedStatesMember
Canada [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Health care cost trend rate assumed for next year | 4.25%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear / clf_RegionReportingInformationByRegionAxis = clf_CanadaMember | 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear / clf_RegionReportingInformationByRegionAxis = clf_CanadaMember

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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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- Definition The ultimate trend rate for health care costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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