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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] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Allocated Share-based Compensation Expense Ìý Ìý Ìý Ìý $ 15,100,000 $ 18,200,000 $ 13,600,000 Ìý
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Ìý Ìý Ìý Ìý $ 0 $ 0 0 Ìý
Performance/vesting period Ìý Ìý Ìý Ìý 3 years Ìý Ìý Ìý
Payout rate, as a percentage of the original grant Ìý Ìý Ìý Ìý 75.30% Ìý Ìý Ìý
Number of performance shares granted Ìý Ìý Ìý Ìý 1,498,386 Ìý Ìý Ìý
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Ìý Ìý Ìý Ìý $ 17,100,000 Ìý Ìý Ìý
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Ìý Ìý Ìý Ìý 1 year Ìý Ìý Ìý
Option Indexed to Issuer's Equity, Strike Price Ìý Ìý Ìý Ìý Ìý $ 13.83 Ìý Ìý
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options Ìý Ìý Ìý Ìý 563,230 Ìý Ìý Ìý
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Ìý Ìý Ìý Ìý $ 10.42 Ìý Ìý Ìý
Common Stock, Shares Authorized Ìý Ìý Ìý Ìý 600,000,000 600,000,000 Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Ìý Ìý Ìý Ìý $ 6.90 $ 6.79 Ìý Ìý
Minimum [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Payout rate, as a percentage of the original grant Ìý Ìý Ìý Ìý 0.00% Ìý Ìý Ìý
Maximum [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Payout rate, as a percentage of the original grant Ìý Ìý Ìý Ìý 200.00% Ìý Ìý Ìý
2015 Equity Plan [Member] | Maximum [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Maximum number of shares that may be issued (in shares) Ìý Ìý Ìý Ìý 12,900,000 Ìý Ìý Ìý
Directors Plan [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Performance/vesting period 1 year 3 years Ìý Ìý Ìý Ìý Ìý Ìý
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued Ìý Ìý Ìý Ìý $ 100,000 $ 100,000 $ 85,000 Ìý
A&R 2015 Equity Plan [Member] | Maximum [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Maximum number of shares that may be issued (in shares) Ìý Ìý Ìý Ìý 15,000,000 Ìý Ìý Ìý
Restricted Stock Units (RSUs) [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Performance/vesting period Ìý Ìý Ìý Ìý 3 years Ìý Ìý Ìý
Restricted Stock Units (RSUs) [Member] | 2015 Equity Plan [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Number of restricted shares granted Ìý Ìý Ìý Ìý Ìý Ìý 3,406,716 Ìý
Restricted Stock Units (RSUs) [Member] | A&R 2015 Equity Plan [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Number of restricted shares granted Ìý Ìý Ìý Ìý 685,599 532,358 Ìý Ìý
Employee Stock Option [Member] | 2012 Equity Plan [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Number of performance shares granted Ìý Ìý Ìý 250,000 Ìý 412,710 Ìý Ìý
Cliffs Natural Resource Inc. 2015 Employee Stock Purchase Plan [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Maximum number of shares that may be issued (in shares) Ìý Ìý Ìý Ìý Ìý Ìý Ìý 10,000,000
January 12, 2015 [Member] | Employee Stock Option [Member] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Grant Date Market Price Ìý Ìý $ 7.70 Ìý Ìý Ìý Ìý Ìý