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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 |  |  |  |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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