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FAIR VALUE OF FINANCIAL INSTRUMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Long-term debt: |  |
Fair Value Adjustment to Interest Rate Hedge |  | $ 0.2
Reported Value Measurement [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | $ 2,113.8 | 2,092.9
Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] |  |
Long-term debt: |  |
Fair Value Adjustment to Interest Rate Hedge | 0.0 | 0.2
Estimate of Fair Value Measurement [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 2,237.0 | 2,039.9
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] |  |
Long-term debt: |  |
Fair Value Adjustment to Interest Rate Hedge | 0.0 | 0.2
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $400 Million 4.875% 2024 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 393.6 | 392.1
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $700 Million 4.875% 2021 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 0.0 | 123.8
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $316.25 Million 1.50% 2025 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 246.7 | 235.2
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $1.075 Billion 5.75% 2025 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 464.2 | 1,048.8
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $750 Million 5.875% 2027 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 716.4 | 0.0
Senior Notes [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $800 Million 6.25% 2040 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 292.9 | 292.8
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $400 Million 4.875% 2024 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 410.0 | 370.2
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $700 Million 4.875% 2021 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 0.0 | 122.3
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $316.25 Million 1.50% 2025 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 385.0 | 352.4
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $1.075 Billion 5.75% 2025 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 473.3 | 962.0
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $750 Million 5.875% 2027 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 718.5 | 0.0
Senior Notes [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] | $800 Million 6.25% 2040 Senior Notes [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 250.2 | 232.8
Line of Credit [Member] | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | ABL Facility [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | 0.0 | 0.0
Line of Credit [Member] | Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | ABL Facility [Member] |  |
Long-term debt: |  |
Long-term Debt, Fair Value | $ 0.0 | $ 0.0

X
- Definition Fair Value Adjustment to Interest Rate Hedge + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Long Term Debt Noncurrent Fair Value [Abstract] + References No definition available. + Details
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Namespace Prefix: | clf_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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