Quarterly report pursuant to Section 13 or 15(d)

DISCONTINUED OPERATIONS DISCONTINUED OPERATIONS (Narrative) (Details)

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DISCONTINUED OPERATIONS DISCONTINUED OPERATIONS (Narrative) (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Mar. 31, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Deconsolidation, Gain (Loss), Amount       $ (528,200,000)    
Canadian Entities [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Discontinued Operation, Tax Effect of Discontinued Operation $ (700,000)   $ 726,700,000 0 $ 807,000,000  
Deconsolidation, Gain (Loss), Amount (13,400,000)   0 (697,400,000) 0  
Cost Method Investments 0     0    
Contingent Liabilities of Deconsolidated Entities 500,000          
Wabush [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Debtor-in-Possession Financing, Amount Arranged 10,000,000     10,000,000    
Debtor-in-Possession Financing, Borrowings Outstanding 4,900,000     4,900,000    
Deconsolidation, Gain (Loss), Amount (13,400,000)          
North American Coal [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Pension and Other Postretirement Defined Benefit Plans, Liabilities 330,000,000     330,000,000    
Discontinued Operation, Tax Effect of Discontinued Operation 2,100,000   $ 278,800,000 2,600,000 $ 326,600,000  
Tangible Asset Impairment Charges   $ 73,400,000        
Bloom Lake Group [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Deconsolidation, Gain (Loss), Amount       818,700,000    
Contingent Liabilities of Deconsolidated Entities       (203,100,000)    
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Tangible Asset Impairment Charges           $ 526,500,000
Other Current Liabilities [Member] | Bloom Lake Group [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Contingent Liabilities recognized in Consolidated Financials 105,500,000     105,500,000    
Other Liabilities [Member] | Bloom Lake Group [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Contingent Liabilities recognized in Consolidated Financials 37,400,000     37,400,000    
Fair Value, Inputs, Level 3 [Member] | Other Current Assets [Member] | Canadian Entities [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Loans, Associated Interest and Accounts Receivable recognized in Consolidated Financials at Fair Value $ 84,500,000     $ 84,500,000