Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE MEASUREMENTS (Narrative) (Details)

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FAIR VALUE MEASUREMENTS (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Fair Value, Assets And Liabilities Components [Line Items]          
Loans Receivable, Fair Value Disclosure $ 7.7   $ 7.7    
Contingent Liabilities recognized in Consolidated Financials 0.0   0.0   $ 37.2
Cash equivalents 77.0   $ 77.0   177.0
Management Estimate of 62% Fe     62.00%    
Derivative Asset 89.5   $ 89.5   33.1
Derivative Asset, Current 89.5   89.5   33.1
Derivative Liability, Current 9.3   9.3   0.5
Derivative Liability 9.3   9.3   0.5
INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX 32.3 $ (2.7) (13.6) $ (0.6)  
Fair Value, Inputs, Level 1 [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Cash equivalents 40.0   40.0   177.0
Derivative Asset 0.0   0.0   0.0
Derivative Liability 0.0   0.0   0.0
Fair Value, Inputs, Level 2 [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Cash equivalents 37.0   37.0   0.0
Derivative Asset 0.0   0.0   1.5
Derivative Liability 0.0   0.0   0.0
Fair Value, Inputs, Level 3 [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Contingent Liabilities recognized in Consolidated Financials 0.0   0.0    
Cash equivalents 0.0   0.0   0.0
Derivative Asset 89.5   89.5   31.6
Derivative Liability 9.3   9.3   0.5
Not Designated as Hedging Instrument [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Derivative Asset 89.5   89.5   33.1
Derivative Liability 9.3   9.3   0.5
Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Customer Supply Agreement [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Derivative Asset, Current 84.8   84.8   21.3
U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Assets [Member] | Not Designated as Hedging Instrument [Member] | Provisional Pricing Arrangements [Member]          
Fair Value, Assets And Liabilities Components [Line Items]          
Derivative Asset, Current $ 4.7   $ 4.7   $ 10.3