Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

v3.3.1.900
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions
 12 Months Ended
Dec. 31, 2015
USD ($)
Facility
T
Dec. 31, 2014
USD ($)
T
Dec. 31, 2013
USD ($)
shares
Jul. 10, 2012
Related Party Transaction [Line Items]        
Deferred Tax Assets, Gross, Current   $ 23,700,000    
Tangible Asset Impairment Charges   537,800,000    
Inventory Valuation Reserves $ 31,800,000 16,000,000    
Trading Day Window Determining Number of Common Shares Issuable on Conversion 20      
Depositary Share Interest in a Share of 7% Series A Mandatory Convertible Preferred Stock, Class A 0.025      
Current Fiscal Year End Date --12-31      
Goodwill $ 2,000,000 2,000,000 $ 74,500,000  
Freight and venture partners' cost reimbursements 180,900,000 278,000,000 259,000,000  
Allowance for Doubtful Accounts Receivable 7,100,000 0    
Provision for Doubtful Accounts $ 7,100,000   0.0  
Derivative, Policy, Percentage of Amount Permitted to be Hedged 75.00%      
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Twelve Months 40.00%      
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Fifteen Months 10.00%      
Discontinued Operation, Tax Effect of Discontinued Operation $ (6,000,000) (1,216,000,000) (184,500,000)  
Asset Impairment Charges 3,300,000 635,500,000 14,300,000  
Goodwill, Impairment Loss 0 73,500,000    
Impairment of Intangible Assets (Excluding Goodwill)   13,800,000    
Deferred Tax Liabilities, Gross, Current   4,000,000    
Unamortized Debt Issuance Expense $ 29,100,000 16,800,000    
Amapa [Member]        
Related Party Transaction [Line Items]        
Equity Method Investment, Other than Temporary Impairment     67,600,000  
U.S. Iron Ore [Member]        
Related Party Transaction [Line Items]        
Number Of Mines | Facility 5      
Goodwill $ 2,000,000 2,000,000 2,000,000  
Freight and venture partners' cost reimbursements $ 157,300,000 $ 271,000,000 259,500,000  
Quantity Of Finished Goods | T 1.3 1.4    
Goodwill, Impairment Loss $ 0 $ 0    
Asia Pacific [Member]        
Related Party Transaction [Line Items]        
Freight and venture partners' cost reimbursements $ 23,600,000 6,900,000    
Asia Pacific Iron Ore [Member]        
Related Party Transaction [Line Items]        
Number Of Mines | Facility 1      
Goodwill $ 0 0 $ 72,500,000  
Goodwill, Impairment Loss $ 0 73,500,000    
Sonoma [Member] | Asia Pacific Coal [Member]        
Related Party Transaction [Line Items]        
Ownership Interest       45.00%
Empire [Member]        
Related Party Transaction [Line Items]        
Noncontrolling Interest, Ownership Percentage by Parent 79.00%      
Bloom Lake [Member]        
Related Party Transaction [Line Items]        
Noncontrolling Interest, Ownership Percentage by Parent     82.80%  
Investments in Ventures [Member] | Amapa [Member]        
Related Party Transaction [Line Items]        
Ownership interest, equity method investment     30.00%  
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member]        
Related Party Transaction [Line Items]        
Number Of Mines | Facility 2      
Take or Pay Contracts [Member]        
Related Party Transaction [Line Items]        
Deferred Revenue   29,300,000    
Customer Supplemental Payments [Member] | U.S. Iron Ore [Member]        
Related Party Transaction [Line Items]        
Deferred Revenue $ 89,900,000 102,800,000    
Deferred Revenue, Current 12,800,000 12,800,000    
Deferred Revenue, Noncurrent $ 77,100,000 90,000,000    
Wisco [Member]        
Related Party Transaction [Line Items]        
Increase in Subsidiary or Equity Method Investee Percentage Ownership Acquired     7.80%  
Units Issued by CQIM to Bloom Lake for Acquisition of Noncontrolling Interest | shares     457,556  
Subsidiary Or Equity Method Investee Percentage Ownership Acquired     17.152%  
Increase/Decrease in Equity for WISCO/Bloom Lake through Acquisition of Noncontrolling Interest     $ 314,800,000  
Bloom Lake [Member]        
Related Party Transaction [Line Items]        
Increase to Cliffs' Shareholders Equity through Acquisition of Noncontrolling Interest   314,800,000    
Minimum [Member] | Other Machinery and Equipment [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 10 years      
Minimum [Member] | Electric Power Facilities [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 10 years      
Minimum [Member] | Land Improvements [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 20 years      
Minimum [Member] | Equipment [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 3 years      
Maximum [Member] | Other Machinery and Equipment [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 45 years      
Maximum [Member] | Electric Power Facilities [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 45 years      
Maximum [Member] | Land Improvements [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 45 years      
Maximum [Member] | Equipment [Member]        
Related Party Transaction [Line Items]        
Property, Plant and Equipment, Useful Life 20 years      
Short-term intercompany loan [Member]        
Related Party Transaction [Line Items]        
Foreign Currency Transaction Gain (Loss), before Tax $ 11,500,000 19,700,000 33,000,000  
Transaction Gains and Losses Resulting from Remeasurement [Member]        
Related Party Transaction [Line Items]        
Foreign Currency Transaction Gain (Loss), before Tax 16,300,000 29,000,000 53,200,000  
Cash and Cash Equivalents [Member]        
Related Party Transaction [Line Items]        
Foreign Currency Transaction Gain (Loss), before Tax $ 1,500,000 $ 10,600,000 $ 20,400,000