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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] Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Grant Date Market Price Ìý Ìý Ìý Ìý $ 7.70 $ 13.83 Ìý Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Ìý Ìý Ìý Ìý 412,710 250,000 Ìý Ìý Ìý
Share-based Payment Arrangement, Option, Exercise Price Range, Shares Exercisable Ìý Ìý Ìý Ìý Ìý Ìý 563,230 Ìý Ìý
Share-based Payment Arrangement, Option, Exercise Price Range, Exercisable, Weighted Average Exercise Price Ìý Ìý Ìý Ìý Ìý Ìý $ 10.42 Ìý Ìý
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Ìý Ìý Ìý Ìý Ìý Ìý $ 16,000,000.0 Ìý Ìý
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Ìý Ìý Ìý Ìý Ìý Ìý 1 year 2 months 12 days Ìý Ìý
Directors Plan [Member] Ìý 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 Ìý
Performance/vesting period 1 year 3 years Ìý Ìý Ìý Ìý Ìý Ìý Ìý
Deferred Compensation Arrangement with Individual, Fair Value of Shares Issued Ìý Ìý Ìý Ìý Ìý Ìý $ 100,000 $ 100,000 $ 100,000
A&R 2015 Equity Plan [Member] | Maximum [Member] Ìý 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 Ìý
Maximum number of shares that may be issued (in shares) Ìý Ìý Ìý Ìý Ìý Ìý 15,000,000.0 Ìý Ìý
Performance Shares [Member] Ìý 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 Ìý
Performance/vesting period Ìý Ìý Ìý Ìý Ìý Ìý 3 years Ìý Ìý
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Ìý Ìý 200.00% 161.20% Ìý Ìý Ìý Ìý Ìý
Performance Shares [Member] | Minimum [Member] Ìý 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 Ìý
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Ìý Ìý Ìý Ìý Ìý Ìý 0.00% Ìý Ìý
Performance Shares [Member] | Maximum [Member] Ìý 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 Ìý
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Ìý Ìý Ìý Ìý Ìý Ìý 200.00% Ìý Ìý
Restricted Stock Units (RSUs) [Member] Ìý 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 Ìý
Performance/vesting period Ìý Ìý Ìý Ìý Ìý Ìý Ìý 3 years Ìý
Cliffs Natural Resource Inc. 2015 Employee Stock Purchase Plan [Member] Ìý 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 Ìý
Maximum number of shares that may be issued (in shares) Ìý Ìý Ìý Ìý Ìý Ìý 10,000,000,000,000 Ìý Ìý