Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Narrative) (Details)

v3.19.3.a.u2
STOCK COMPENSATION PLANS (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Apr. 25, 2018
Apr. 23, 2018
Jun. 26, 2017
Feb. 21, 2017
Mar. 31, 2015
Dec. 31, 2014
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Grant Date Market Price         $ 7.70 $ 13.83      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross         412,710 250,000      
Share-based Payment Arrangement, Option, Exercise Price Range, Shares Exercisable             563,230    
Share-based Payment Arrangement, Option, Exercise Price Range, Exercisable, Weighted Average Exercise Price             $ 10.42    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount             $ 16,000,000.0    
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition             1 year 2 months 12 days    
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 $ 100,000
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)             15,000,000.0    
Performance Shares [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Performance/vesting period             3 years    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage     200.00% 161.20%          
Performance Shares [Member] | Minimum [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage             0.00%    
Performance Shares [Member] | Maximum [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage             200.00%    
Restricted Stock Units (RSUs) [Member]                  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                  
Performance/vesting period               3 years  
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)             10,000,000,000,000