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FAIR VALUE MEASUREMENTS - Quantitative Inputs And Assumptions For Level 3 Assets And Liabilities (Details) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($)
Managements Estimate Of Sixty Two Percent [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Provisional Pricing Arrangements [Member] | Weighted Average [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Measurement With Unobservable Inputs Range | 94 |
Atlantic Basin Pellet Premium [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Provisional Pricing Arrangements [Member] | Weighted Average [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Measurement With Unobservable Inputs Range | 32 |
Customer's Hot-Rolled Steel Estimate [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Customer Supply Agreement [Member] | Minimum [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Measurement With Unobservable Inputs Range | 562 |
Customer's Hot-Rolled Steel Estimate [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Customer Supply Agreement [Member] | Maximum [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Measurement With Unobservable Inputs Range | 639 |
Customer's Hot-Rolled Steel Estimate [Member] | Fair Value, Inputs, Level 3 [Member] | Not Designated as Hedging Instrument [Member] | Valuation, Market Approach [Member] | Customer Supply Agreement [Member] | Weighted Average [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value, Measurement With Unobservable Inputs Range | 564 |
Other Current Assets [Member] | Customer Supply Agreement [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | $ 27.3 | $ 44.5
Other Current Assets [Member] | Provisional Pricing Arrangements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 8.0 | 1.3
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 27.3 | 44.5
Other Current Assets [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 8.0 | 1.3
Other Current Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Customer Supply Agreement [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 27.3 | 44.5
Other Current Assets [Member] | Fair Value, Inputs, Level 3 [Member] | Provisional Pricing Arrangements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | $ 8.0 | $ 1.3

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