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v3.10.0.1

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | 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 |
United Taconite [Member] |  |
Related Party Transaction [Line Items] |  |
Entity Address, State or Province | Minnesota |
Tilden [Member] |  |
Related Party Transaction [Line Items] |  |
Entity Address, State or Province | Michigan |
Noncontrolling Interest, Ownership Percentage by Parent |  | 100.00%
Empire [Member] |  |
Related Party Transaction [Line Items] |  |
Entity Address, State or Province | Michigan |
Noncontrolling Interest, Ownership Percentage by Parent |  | 100.00%

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

X
- 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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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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
Name: | us-gaap_RelatedPartyTransactionLineItems
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: |