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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
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12 Months Ended |
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Dec. 31, 2014
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Dec. 31, 2012
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| Northshore [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Minnesota
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_NorthshoreMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| United Taconite [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Minnesota
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_UnitedTaconiteMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Wabush [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Newfoundland and Labrador/ Québec, Canada
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_WabushMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Bloom Lake [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Québec, Canada
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| Noncontrolling Interest, Ownership Percentage by Parent |
82.80%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BloomLakeMember
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75.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BloomLakeMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Tilden [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Michigan
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| Noncontrolling Interest, Ownership Percentage by Parent |
85.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_TildenMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Empire [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Michigan
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| Noncontrolling Interest, Ownership Percentage by Parent |
79.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_EmpireMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Koolyanobbing [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Western Australia
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_KoolyanobbingMember
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| Segment Reporting Information, Description of Products and Services |
Iron Ore
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| Pinnacle [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
West Virginia
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_PinnacleMember
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| Segment Reporting Information, Description of Products and Services |
Coal
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| Oak Grove [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Alabama
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_OakGroveMember
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| Segment Reporting Information, Description of Products and Services |
Coal
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| CLCC [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
West Virginia
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_ClccMember
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| Segment Reporting Information, Description of Products and Services |
Coal
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| Black Label Deposit [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Ontario, Canada
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BlackLabelDepositMember
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| Segment Reporting Information, Description of Products and Services |
Chromite
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| Black Thor Deposit [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Ontario, Canada
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| Noncontrolling Interest, Ownership Percentage by Parent |
100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BlackThorDepositMember
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| Segment Reporting Information, Description of Products and Services |
Chromite
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| Big Daddy Chromite Deposit [Member] |
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| Related Party Transaction [Line Items] |
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| Entity Address, State or Province |
Ontario, Canada
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| Noncontrolling Interest, Ownership Percentage by Parent |
70.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BigDaddyChromiteDepositMember
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| Segment Reporting Information, Description of Products and Services |
Chromite
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