Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v3.6.0.2
STOCK COMPENSATION PLANS (Narrative) (Details)
 12 Months Ended
Dec. 31, 2016
USD ($)
Agreement
Plan
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2014
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 | $ $ 14,200,000 $ 13,900,000 $ 21,500,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | $ $ 0 $ 0 $ 7,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plan Year Agreements | Agreement 3    
Number of performance shares granted 3,571,337    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 14,700,000    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 9 months 15 days    
Option Indexed to Issuer's Equity, Strike Price | $ / shares   $ 13.83  
Common Stock, Shares Authorized 400,000,000 400,000,000  
2015 Equity Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance/vesting period 27 months    
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]      
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]      
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued | $ $ 85,000    
2012 Equity Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Performance/vesting period 3 years    
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] | 2012 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] | 2012 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]      
Performance/vesting period 3 years    
Number of restricted shares granted 3,400,000 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