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SHAREHOLDERS' EQUITY (Narrative (Details) | Sep. 30, 2018
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Noncontrolling Interest, Ownership Percentage by Parent | 100.00%
Empire |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%
Tilden |
Noncontrolling Interest, Ownership Percentage by Parent | 100.00%

X
- 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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- Details
Name: | clf_NameofOperationAxis=clf_EmpireMember
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: |