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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details) | 3 Months Ended
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Mar. 31, 2015
Northshore [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Minnesota
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_NorthshoreMember
Segment Reporting Information, Description of Products and Services | Iron Ore
United Taconite [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Minnesota
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_UnitedTaconiteMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Wabush [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Newfoundland and Labrador/ Quebec, Canada
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_WabushMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Bloom Lake [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Quebec, Canada
Noncontrolling Interest, Ownership Percentage by Parent | 82.80%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_BloomLakeMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Tilden [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Michigan
Noncontrolling Interest, Ownership Percentage by Parent | 85.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_TildenMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Empire [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Michigan
Noncontrolling Interest, Ownership Percentage by Parent | 79.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_EmpireMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Koolyanobbing [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Western Australia
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_KoolyanobbingMember
Segment Reporting Information, Description of Products and Services | Iron Ore
Pinnacle [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | West Virginia
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_PinnacleMember
Segment Reporting Information, Description of Products and Services | Coal
Oak Grove [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Alabama
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_OakGroveMember
Segment Reporting Information, Description of Products and Services | Coal
Black Label Deposit [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_BlackLabelDepositMember
Segment Reporting Information, Description of Products and Services | Chromite
Black Thor Deposit [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_BlackThorDepositMember
Segment Reporting Information, Description of Products and Services | Chromite
Big Daddy Chromite Deposit [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent | 70.00%us-gaap_MinorityInterestOwnershipPercentageByParent / clf_NameofOperationAxis = clf_BigDaddyChromiteDepositMember
Segment Reporting Information, Description of Products and Services | Chromite

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- Definition Name of the state or province. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Description of the types of products and services from which the reportable segment derives its revenue. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8721-108599 + Details
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- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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Period Type: | duration

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Name: | clf_NameofOperationAxis=clf_BigDaddyChromiteDepositMember
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- Details
Name: | clf_NameofOperationAxis=clf_BlackLabelDepositMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_BlackThorDepositMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_BloomLakeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_EmpireMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_KoolyanobbingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_NorthshoreMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_OakGroveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_PinnacleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_TildenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_UnitedTaconiteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | clf_NameofOperationAxis=clf_WabushMember
Namespace Prefix: |
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Balance Type: |
Period Type: |