Annual report pursuant to Section 13 and 15(d)

STOCK COMPENSATION PLANS (Schedule of Compensation Costs) (Details)

v3.10.0.1
STOCK COMPENSATION PLANS (Schedule of Compensation Costs) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Selling, general and administrative expenses                 $ (116.8) $ (102.9) $ (115.8)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES                 564.7 108.2 110.4
Income Tax Expense (Benefit)                 (475.2) (252.4) (12.2)
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ 609.5 $ 437.8 $ 165.1 $ (84.3) $ 309.9 $ 53.4 $ 31.8 $ (28.1) $ 1,128.1 $ 367.0 $ 174.1
Earnings per common share attributable to Cliffs common shareholders - basic: $ 2.06 $ 1.47 $ 0.55 $ (0.29) $ 1.04 $ 0.18 $ 0.10 $ (0.11) $ 3.80 $ 1.28 $ 0.88
Earnings per common share attributable to Cliffs common shareholders - diluted: $ 1.98 $ 1.41 $ 0.55 $ (0.29) $ 1.03 $ 0.18 $ 0.10 $ (0.11) $ 3.71 $ 1.26 $ 0.87
Deferred Compensation, Share-based Payments [Member]                      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                      
Costs and Expenses                 $ 1.7 $ 1.9 $ 1.8
Selling, general and administrative expenses                 (13.4) (16.3) (11.8)
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES                 15.1 18.2 13.6
Income Tax Expense (Benefit)                 0.0 0.0 0.0
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS                 $ 15.1 $ 18.2 $ 13.6
Earnings per common share attributable to Cliffs common shareholders - basic:                 $ 0.05 $ 0.06 $ 0.07
Earnings per common share attributable to Cliffs common shareholders - diluted:                 $ 0.05 $ 0.06 $ 0.07