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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 |  |

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