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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details) | 9 Months Ended
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Sep. 30, 2018
Northshore |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Minnesota
United Taconite |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Minnesota
Tilden |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Michigan
Empire |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Michigan
Koolyanobbing |
Related Party Transaction [Line Items] |
Entity Address, State or Province | Western Australia

X
- Definition Name of the state or province. + References No definition available. + Details
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Namespace Prefix: | dei_
Data Type: | xbrli:normalizedStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

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

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

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