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SEGMENT REPORTING - Segment Capital Additions (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Segment Reporting Information [Line Items] |  |  |  |
Property, Plant and Equipment, Additions | $ 73.0 | $ 191.6 | $ 230.7 | $ 320.9
Steel and Manufacturing [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Property, Plant and Equipment, Additions | 55.6 | 155.1 | 178.8 | 237.5
Mining and Pelletizing [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Property, Plant and Equipment, Additions | 17.4 | 35.6 | 51.6 | 82.4
Corporate and Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Property, Plant and Equipment, Additions | $ 0.0 | $ 0.9 | $ 0.3 | $ 1.0

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- Definition Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=clf_MiningandPelletizingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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