Annual report pursuant to Section 13 and 15(d)

EARNINGS PER SHARE (Earnings Per Share Computation) (Details)

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EARNINGS PER SHARE (Earnings Per Share Computation) (Details) - USD ($)
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
Debt Instrument [Line Items]                      
INCOME FROM CONTINUING OPERATIONS         $ 333,000,000 $ 22,300,000 $ 83,800,000 $ (78,500,000) $ 1,039,900,000 $ 360,600,000 $ 122,600,000
Net Income (Loss) Attributable to Noncontrolling Interest                 0 (3,900,000) 25,200,000
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Noncontrolling Interest         0 500,000 1,700,000 1,700,000      
Income (Loss) from Continuing Operations Attributable to Parent $ 624,100,000 $ 199,800,000 $ 229,400,000 $ (13,400,000) 333,000,000 22,800,000 85,500,000 (76,800,000) 1,039,900,000 364,500,000 97,400,000
INCOME FROM DISCONTINUED OPERATIONS, net of tax                 88,200,000 2,500,000 76,700,000
INCOME FROM DISCONTINUED OPERATIONS, net of tax (14,600,000) 238,000,000 (64,300,000) (70,900,000) (23,100,000) 30,600,000 (53,700,000) 48,700,000 88,200,000 2,500,000 76,700,000
Net income attributable to Cliffs shareholders $ 609,500,000 $ 437,800,000 $ 165,100,000 $ (84,300,000) $ 309,900,000 $ 53,400,000 $ 31,800,000 $ (28,100,000) $ 1,128,100,000 $ 367,000,000 $ 174,100,000
Weighted average number of shares:                      
Basic                 297,200,000 288,400,000 197,700,000
$316.5 million 1.50% 2025 Convertible Senior Notes                 3,400,000 0 0
Employee stock plans                 3,500,000 4,600,000 2,400,000
Diluted                 304,100,000 293,000,000 200,100,000
Earnings per common share attributable to Cliffs common shareholders - basic:                      
Continuing operations $ 2.11 $ 0.67 $ 0.77 $ (0.05) $ 1.12 $ 0.08 $ 0.28 $ (0.29) $ 3.50 $ 1.27 $ 0.49
Discontinued operations (0.05) 0.80 (0.22) (0.24) (0.08) 0.10 (0.18) 0.18 0.30 0.01 0.39
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:                      
Continuing operations 2.03 0.64 0.76 (0.05) 1.11 0.08 0.28 (0.29) 3.42 1.25 0.49
Discontinued operations (0.05) 0.77 (0.21) (0.24) (0.08) 0.10 (0.18) 0.18 0.29 0.01 0.38
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
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount       3,800,000.0       4,600,000.0   0  
$316 Million 1.5% 2025 Senior Notes [Domain]                      
Debt Instrument [Line Items]                      
Face amount $ 316,250,000.00               $ 316,250,000.00    
Debt Instrument, Interest Rate, Stated Percentage 1.50%               1.50%