Annual report pursuant to Section 13 and 15(d)

NEW ACCOUNTING STANDARDS (Schedule of New Accounting Pronouncements and Changes in Accounting Principles) (Details)

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NEW ACCOUNTING STANDARDS (Schedule of New Accounting Pronouncements and Changes in Accounting Principles) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash and cash equivalents $ 823.2 $ 978.3 $ 978.3    
Accounts receivable, net 226.7 183.3 106.7    
Inventories 87.9 87.0 138.4    
Supplies and other inventories 93.2 88.8 88.8    
Derivative assets 91.5 49.5 37.9    
Income tax receivable, current 117.3 13.3 13.3    
Loans to and accounts receivables from the Canadian Entities 0.0 51.6 51.6    
Current assets of discontinued operations 12.4 118.5 118.5    
Other current assets 27.4 11.1 11.1    
TOTAL CURRENT ASSETS 1,479.6 1,581.4 1,544.6    
PROPERTY, PLANT AND EQUIPMENT, NET 1,286.0 1,033.8 1,033.8    
Deposits for property, plant and equipment 83.0 17.8 17.8    
Income tax receivable, non-current 121.3 235.3 235.3    
Non-current assets of discontinued operations 0.0 20.3 20.3    
Other non-current assets 94.9 101.6 101.6    
TOTAL OTHER ASSETS 764.0 375.0 375.0    
TOTAL ASSETS 3,529.6 2,990.2 2,953.4 $ 1,923.9  
Accounts payable 186.8 100.9 99.5    
Accrued employment costs 74.0 52.7 52.7    
State and local taxes payable 35.5 30.2 30.2    
Accrued interest 38.4 31.4 31.4    
Contingent claims 0.0 55.6 55.6    
Partnership distribution payable 43.5 44.2 44.2    
Current liabilities of discontinued operations 6.7 75.0 75.0    
Other current liabilities 83.3 65.0 63.6    
TOTAL CURRENT LIABILITIES 468.2 455.0 452.2    
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES 248.7 257.7 257.7    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 172.0 167.7 167.7    
LONG-TERM DEBT 2,092.9 2,304.2 2,304.2    
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS 8.3 52.2 52.2    
OTHER LIABILITIES 115.3 163.5 163.5    
TOTAL LIABILITIES 3,105.4 3,400.3 3,397.5    
TOTAL CLIFFS SHAREHOLDERS' EQUITY (DEFICIT) 424.2 (410.3) (444.3)    
NONCONTROLLING INTEREST 0.0 0.2 0.2    
TOTAL EQUITY (DEFICIT) 424.2 (410.1) (444.1) $ (1,330.5) $ (1,811.6)
TOTAL LIABILITIES AND EQUITY (DEFICIT) 3,529.6 2,990.2 $ 2,953.4    
Calculated under Revenue Guidance in Effect before Topic 606 [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash and cash equivalents 823.2        
Accounts receivable, net 108.7        
Inventories 141.3        
Supplies and other inventories 93.2        
Derivative assets 60.7        
Income tax receivable, current 117.3        
Current assets of discontinued operations 12.4        
Other current assets 27.4        
TOTAL CURRENT ASSETS 1,384.2        
PROPERTY, PLANT AND EQUIPMENT, NET 1,286.0        
Deposits for property, plant and equipment 83.0        
Income tax receivable, non-current 121.3        
Other non-current assets 94.9        
TOTAL OTHER ASSETS 776.3        
TOTAL ASSETS 3,446.5        
Accounts payable 184.9        
Accrued employment costs 74.0        
State and local taxes payable 35.5        
Accrued interest 38.4        
Partnership distribution payable 43.5        
Current liabilities of discontinued operations 6.7        
Other current liabilities 83.7        
TOTAL CURRENT LIABILITIES 466.7        
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES 248.7        
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 172.0        
LONG-TERM DEBT 2,092.9        
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS 8.3        
OTHER LIABILITIES 115.3        
TOTAL LIABILITIES 3,103.9        
TOTAL CLIFFS SHAREHOLDERS' EQUITY (DEFICIT) 342.6        
TOTAL LIABILITIES AND EQUITY (DEFICIT) 3,446.5        
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash and cash equivalents 0.0 0.0      
Accounts receivable, net 118.0 76.6      
Inventories (53.4) (51.4)      
Supplies and other inventories 0.0 0.0      
Derivative assets 30.8 11.6      
Income tax receivable, current 0.0 0.0      
Loans to and accounts receivables from the Canadian Entities   0.0      
Current assets of discontinued operations 0.0 0.0      
Other current assets 0.0 0.0      
TOTAL CURRENT ASSETS 95.4 36.8      
PROPERTY, PLANT AND EQUIPMENT, NET 0.0 0.0      
Deposits for property, plant and equipment 0.0 0.0      
Income tax receivable, non-current 0.0 0.0      
Non-current assets of discontinued operations   0.0      
Other non-current assets 0.0 0.0      
TOTAL OTHER ASSETS (12.3) 0.0      
TOTAL ASSETS 83.1 36.8      
Accounts payable 1.9 1.4      
Accrued employment costs 0.0 0.0      
State and local taxes payable 0.0 0.0      
Accrued interest 0.0 0.0      
Contingent claims   0.0      
Partnership distribution payable 0.0 0.0      
Current liabilities of discontinued operations 0.0 0.0      
Other current liabilities (0.4) 1.4      
TOTAL CURRENT LIABILITIES 1.5 2.8      
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES 0.0 0.0      
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 0.0 0.0      
LONG-TERM DEBT 0.0 0.0      
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS 0.0 0.0      
OTHER LIABILITIES 0.0 0.0      
TOTAL LIABILITIES 1.5 2.8      
TOTAL CLIFFS SHAREHOLDERS' EQUITY (DEFICIT) 81.6 34.0      
NONCONTROLLING INTEREST   0.0      
TOTAL EQUITY (DEFICIT)   34.0      
TOTAL LIABILITIES AND EQUITY (DEFICIT) $ 83.1 $ 36.8