Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
NEW ACCOUNTING STANDARDS (Schedule of New Accounting Pronouncements and Changes in Accounting Principles) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash and cash equivalents $ 786.6 $ 295.3 $ 1,007.7 $ 1,007.7  
Accounts receivable, net 47.2   217.2 140.6  
Inventories (324.4)   (132.0) (183.4)  
Supplies and other inventories 81.7   93.9 93.9  
Derivative assets 93.6   51.0 39.4  
Loans to and accounts receivable from the Canadian Entities 50.4   51.6 51.6  
Other current assets 28.5   28.0 28.0  
Assets, Current 1,412.4   1,581.4 1,544.6  
PROPERTY, PLANT AND EQUIPMENT, NET 1,047.3   1,051.0 1,051.0  
Deposits for property, plant and equipment 74.1   17.8 17.8  
Income tax receivable 219.9   235.3 235.3  
Other non-current assets 109.2   104.7 104.7  
Total Other Assets 403.2   357.8 357.8  
Assets 2,862.9   2,990.2 2,953.4  
Accounts payable 99.5   129.1 127.7  
Accrued expenses 94.4   107.1 107.1  
Accrued interest 28.2   31.4 31.4  
Contingent claims 54.3   55.6 55.6  
Partnership distribution payable 44.2   44.2 44.2  
Other current liabilities 104.3   87.6 86.2  
Liabilities, Current 424.9   455.0 452.2  
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 251.4   257.7 257.7  
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 181.2   196.5 196.5  
LONG-TERM DEBT 2,308.2   2,304.2 2,304.2  
OTHER LIABILITIES 182.0   186.9 186.9  
Liabilities 3,347.7   3,400.3 3,397.5  
Stockholders' Equity Attributable to Parent (485.0)   (410.3) (444.3)  
Stockholders' Equity Attributable to Noncontrolling Interest 0.2   0.2 0.2  
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (484.8) (703.0) (410.1) (444.1) $ (1,330.5)
Liabilities and Equity 2,862.9   2,990.2 $ 2,953.4  
Product (220.7) (412.8)      
Freight and venture partners' cost reimbursements (18.3) (48.8)      
Revenues 239.0 461.6      
Cost of Goods and Services Sold (242.6) (365.3)      
Sales margin (3.6) 96.3      
Selling, General and Administrative Expense (27.7) (27.7)      
Miscellaneous - net (8.7) 11.5      
Other operating expense (36.4) (16.2)      
Operating Income (Loss) (40.0) 80.1      
Interest expense, net (33.5) (42.8)      
Other non-operating income 4.4 2.5      
Other expense (29.1) (112.2)      
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest (69.1) (32.1)      
Income Tax Expense (Benefit) (15.7) 1.8      
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest (84.8) (30.3)      
INCOME FROM DISCONTINUED OPERATIONS, NET OF TAX 0.5 0.5      
Net Loss (84.3) (29.8)      
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST 0.0 1.7      
NET LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ (84.3) $ (28.1)      
Income (Loss) from Continuing Operations, Per Basic Share $ (0.29) $ (0.11)      
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share 0.00 0.00      
Earnings Per Share, Basic (0.29) (0.11)      
Income (Loss) from Continuing Operations, Per Diluted Share (0.29) (0.11)      
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share 0.00 0.00      
Earnings Per Share, Diluted $ (0.29) $ (0.11)      
Basic 297,266 265,164      
Diluted 297,266 265,164      
Scenario, Previously Reported [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cost of Goods and Services Sold   $ (365.9)      
Selling, General and Administrative Expense   (25.7)      
Miscellaneous - net   11.9      
Operating Income (Loss)   81.9      
Other non-operating income   0.7      
Net Loss   (29.8)      
Accounting Standards Update 2017-07 [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cost of Goods and Services Sold   0.6      
Selling, General and Administrative Expense   (2.0)      
Miscellaneous - net   (0.4)      
Operating Income (Loss)   (1.8)      
Other non-operating income   1.8      
Net Loss   $ 0.0      
Calculated under Revenue Guidance in Effect before Topic 606 [Member]          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]          
Cash and cash equivalents $ 786.6        
Accounts receivable, net 24.9        
Inventories (332.0)        
Supplies and other inventories 81.7        
Derivative assets 91.3        
Loans to and accounts receivable from the Canadian Entities 50.4        
Other current assets 28.5        
Assets, Current 1,395.4        
PROPERTY, PLANT AND EQUIPMENT, NET 1,047.3        
Deposits for property, plant and equipment 74.1        
Income tax receivable 219.9        
Other non-current assets 109.2        
Total Other Assets 403.2        
Assets 2,845.9        
Accounts payable 99.2        
Accrued expenses 94.4        
Accrued interest 28.2        
Contingent claims 54.3        
Partnership distribution payable 44.2        
Other current liabilities 104.0        
Liabilities, Current 424.3        
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 251.4        
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 181.2        
LONG-TERM DEBT 2,308.2        
OTHER LIABILITIES 182.0        
Liabilities 3,347.1        
Stockholders' Equity Attributable to Parent (501.4)        
Stockholders' Equity Attributable to Noncontrolling Interest 0.2        
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (501.2)        
Liabilities and Equity 2,845.9        
Product (279.1)        
Freight and venture partners' cost reimbursements (22.4)        
Revenues 301.5        
Cost of Goods and Services Sold (286.2)        
Sales margin 15.3        
Selling, General and Administrative Expense (27.7)        
Miscellaneous - net (8.7)        
Other operating expense (36.4)        
Operating Income (Loss) (21.1)        
Interest expense, net (33.5)        
Other non-operating income 4.4        
Other expense (29.1)        
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest (50.2)        
Income Tax Expense (Benefit) (15.7)        
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest (65.9)        
INCOME FROM DISCONTINUED OPERATIONS, NET OF TAX 0.5        
Net Loss (65.4)        
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST 0.0        
NET LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ (65.4)        
Income (Loss) from Continuing Operations, Per Basic Share $ (0.23)        
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share 0.00        
Earnings Per Share, Basic (0.23)        
Income (Loss) from Continuing Operations, Per Diluted Share (0.23)        
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share 0.00        
Earnings Per Share, Diluted $ (0.23)        
Basic 297,266        
Diluted 297,266        
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 22.3   76.6    
Inventories (7.6)   (51.4)    
Supplies and other inventories 0.0   0.0    
Derivative assets 2.3   11.6    
Loans to and accounts receivable from the Canadian Entities 0.0   0.0    
Other current assets 0.0   0.0    
Assets, Current 17.0   36.8    
PROPERTY, PLANT AND EQUIPMENT, NET 0.0   0.0    
Deposits for property, plant and equipment 0.0   0.0    
Income tax receivable 0.0   0.0    
Other non-current assets 0.0   0.0    
Total Other Assets 0.0   0.0    
Assets 17.0   36.8    
Accounts payable 0.3   1.4    
Accrued expenses 0.0   0.0    
Accrued interest 0.0   0.0    
Contingent claims 0.0   0.0    
Partnership distribution payable 0.0   0.0    
Other current liabilities 0.3   1.4    
Liabilities, Current 0.6   2.8    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 0.0   0.0    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 0.0   0.0    
LONG-TERM DEBT 0.0   0.0    
OTHER LIABILITIES 0.0   0.0    
Liabilities 0.6   2.8    
Stockholders' Equity Attributable to Parent 16.4   34.0    
Stockholders' Equity Attributable to Noncontrolling Interest 0.0   0.0    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 16.4        
Liabilities and Equity 17.0   $ 36.8    
Product (58.4)        
Freight and venture partners' cost reimbursements (4.1)        
Revenues 62.5        
Cost of Goods and Services Sold (43.6)        
Sales margin (18.9)        
Selling, General and Administrative Expense 0.0        
Miscellaneous - net 0.0        
Other operating expense 0.0        
Operating Income (Loss) (18.9)        
Interest expense, net 0.0        
Other non-operating income 0.0        
Other expense 0.0        
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest (18.9)        
Income Tax Expense (Benefit) 0.0        
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest (18.9)        
INCOME FROM DISCONTINUED OPERATIONS, NET OF TAX 0.0        
Net Loss (18.9)        
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST 0.0        
NET LOSS ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ (18.9)        
Income (Loss) from Continuing Operations, Per Basic Share $ (0.06)        
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share 0.00        
Earnings Per Share, Basic (0.06)        
Income (Loss) from Continuing Operations, Per Diluted Share (0.06)        
Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share 0.00        
Earnings Per Share, Diluted $ (0.06)