Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENTS - Quantitative Inputs And Assumptions For Level 3 Assets And Liabilities (Details)

v3.20.2
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