Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v3.8.0.1
STOCK COMPENSATION PLANS (Narrative) (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
Agreement
Plan
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
shares
Jun. 26, 2017
$ / shares
Feb. 21, 2017
$ / shares
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 | $ $ 18,700,000 $ 14,200,000 $ 13,900,000    
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense | $ $ 0 $ 0 0    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Plan Year Agreements | Agreement 3        
Payout rate, as a percentage of the original grant 75.30%        
Number of performance shares granted 1,999,562        
Number of restricted shares granted 400,000        
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $ $ 20,100,000        
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 1 year 6 months        
Option Indexed to Issuer's Equity, Strike Price | $ / shares   $ 13.83      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options 599,870        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price | $ / shares $ 10.25        
Common Stock, Shares Authorized 600,000,000 400,000,000      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares $ 6.79 $ 5.55   $ 10.74 $ 19.69
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        
Directors Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued | $ $ 100,000 $ 85,000 $ 85,000    
2012 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance/vesting period 3 years        
A&R 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) 15,000,000        
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 553,725 3,406,716 1,473,184    
Restricted Stock Units (RSUs) [Member] | 2012 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of restricted shares granted     874,575    
Restricted Stock Units (RSUs) [Member] | A&R 2015 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of restricted shares granted 532,358        
Employee Stock Option [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of performance shares granted 0 0 412,710    
Employee Stock Option [Member] | 2015 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of performance shares granted 0 0 0    
Employee Stock Option [Member] | 2012 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of performance shares granted     412,710    
Employee Stock Option [Member] | A&R 2015 Equity Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of performance shares granted 0        
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        
January 12, 2015 [Member] | Employee Stock Option [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Grant Date Market Price | $ / shares $ 7.70