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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details) | 12 Months Ended
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Dec. 31, 2017
Related Party Transaction [Line Items] |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%
Northshore [Member] |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Minnesota
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%
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%
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 | 100.00%
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 | 100.00%
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%
Segment Reporting Information, Description of Products and Services | Iron Ore

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- Definition Name of the state or province. + References No definition available. + Details
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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=68060357&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
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- Details
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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_TildenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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- Details
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Period Type: |