Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v3.3.1.900
STOCK COMPENSATION PLANS (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
shares
Sep. 30, 2014
USD ($)
Sep. 30, 2015
Dec. 31, 2015
USD ($)
Agreement
Plan
shares
Dec. 31, 2014
USD ($)
$ / shares
shares
Dec. 31, 2013
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plans | Plan       2    
Allocated Share-based Compensation Expense | $       $ 13,900,000 $ 21,500,000 $ 19,100,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | $       $ 0 $ 7,500,000 $ 6,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plan Year Agreements | Agreement       3    
Number of performance shares granted       3,769,700    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $       $ 23,500,000    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition       2 years 6 months 20 days    
Option Indexed to Issuer's Equity, Strike Price | $ / shares         $ 13.83  
Common Stock, Shares Authorized 400,000,000     400,000,000 400,000,000  
2015 Equity Plan [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Maximum number of shares that may be issued (in shares)       12,900,000    
Two Thousand Twelve Equity Plan and Amended Equity Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Performance/vesting period     3 years      
Payout rate, as a percentage of the original grant           23.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Available for Issuance Proposed to be added to the Plan       5,000,000    
Directors Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Stock Issued During Period Value Director Stock Award | $ $ 42,500 $ 42,500     $ 85,000  
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued | $       $ 85,000 250,000  
Directors Plan [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Maximum number of shares that may be issued (in shares)       300,000    
2012 Equity Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares granted under the award       900,000    
Number of performance shares granted       900,000    
Number of restricted shares granted       400,000    
Change in Control [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Allocated Share-based Compensation Expense | $         $ 11,700,000  
2015 to 2017 Performance Period [Member] | Two Thousand Twelve Equity Plan and Amended Equity Plan [Member] | Minimum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Payout rate, as a percentage of the original grant     0.00%      
2015 to 2017 Performance Period [Member] | Two Thousand Twelve Equity Plan and Amended Equity Plan [Member] | Maximum [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Payout rate, as a percentage of the original grant     200.00%      
Restricted Stock Units (RSUs) [Member] | 2015 Equity Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of restricted shares granted       1,500,000    
Cliffs Natural Resource Inc. 2015 Employee Stock Purchase Plan [Member]            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Common Stock, Shares Authorized       10,000,000