Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v3.10.0.1
STOCK COMPENSATION PLANS (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Apr. 25, 2018
Apr. 23, 2018
Mar. 31, 2015
Dec. 31, 2014
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 09, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Allocated Share-based Compensation Expense         $ 15,100,000 $ 18,200,000 $ 13,600,000  
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense         $ 0 $ 0 0  
Performance/vesting period         3 years      
Payout rate, as a percentage of the original grant         75.30%      
Number of performance shares granted         1,498,386      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized         $ 17,100,000      
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition         1 year      
Option Indexed to Issuer's Equity, Strike Price           $ 13.83    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options         563,230      
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price         $ 10.42      
Common Stock, Shares Authorized         600,000,000 600,000,000    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value         $ 6.90 $ 6.79    
Minimum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Payout rate, as a percentage of the original grant         0.00%      
Maximum [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Payout rate, as a percentage of the original grant         200.00%      
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]                
Performance/vesting period 1 year 3 years            
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued         $ 100,000 $ 100,000 $ 85,000  
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      
Restricted Stock Units (RSUs) [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Performance/vesting period         3 years      
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             3,406,716  
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         685,599 532,358    
Employee Stock Option [Member] | 2012 Equity Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Number of performance shares granted       250,000   412,710    
Cliffs Natural Resource Inc. 2015 Employee Stock Purchase Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Maximum number of shares that may be issued (in shares)               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     $ 7.70