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STOCK COMPENSATION PLANS (Narrative) (Details) | 12 Months Ended |  |
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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 |  |  |  |

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