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GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES (Amortization of Intangible Liability) (Details) $ in Millions | Mar. 31, 2020 USD ($)
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Goodwill and Intangible Assets Disclosure [Abstract] |
Finite-Lived Intangible Liabilities, Amortization Credit, Remainder of Fiscal Year | $ 7.0
Finite-Lived Intangible Liabilities, Amortization Credit, Next Twelve Months | 10.9
Finite-Lived Intangible Liabilities, Amortization Credit, Year Two | 10.9
Finite-Lived Intangible Liabilities, Amortization Credit, Year Three | 10.9
Finite-Lived Intangible Liabilities, Amortization Credit, Year Four | 10.9
Finite-Lived Intangible Liabilities, Amortization Credit, Year Five | $ 10.9

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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Next Twelve Months + References No definition available. + Details
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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Remainder of Fiscal Year + References No definition available. + Details
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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Year Five + References No definition available. + Details
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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Year Four + References No definition available. + Details
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Balance Type: | credit
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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Year Three + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Finite-Lived Intangible Liabilities, Amortization Credit, Year Two + References No definition available. + Details
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Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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