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 $)
In Millions, except Share data in Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Earnings Per Share [Abstract]                      
Net Income (Loss) from Continuing Operations attributable to Cliffs shareholders $ (1,649.1) $ 87.8 $ 255.7 $ 370.3 $ 168.9 $ 618.0 $ 392.8 $ 419.3 $ (935.3) $ 1,599.0 $ 997.4
Income (Loss) and Gain on Sale from Discontinued Operations, net of tax 30.8 (2.7) 2.3 5.5 16.5 (16.8) 16.3 4.1 35.9 20.1 22.5
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS $ (1,618.3) $ 85.1 $ 258.0 $ 375.8 $ 185.4 $ 601.2 $ 409.1 $ 423.4 $ (899.4) $ 1,619.1 $ 1,019.9
Weighted Average Number of Shares:                      
Basic (in shares)                 142,351 140,234 135,301
Employee stock plans (in shares)                 0 778 837
Diluted (in shares)                 142,351 141,012 136,138
Earnings (loss) per Common Share Attributable to Cliffs Shareholders - Basic:                      
Continuing operations (in usd per share) $ (11.58) $ 0.62 $ 1.79 $ 2.60 $ 1.19 $ 4.29 $ 2.82 $ 3.09 $ (6.57) $ 11.41 $ 7.37
Discontinued operations (in usd per share) $ 0.22 $ (0.02) $ 0.02 $ 0.04 $ 0.11 $ (0.12) $ 0.12 $ 0.03 $ 0.25 $ 0.14 $ 0.17
Earnings per common share attributable to Cliffs shareholders - Basic (in usd per share) $ (11.36) $ 0.60 $ 1.81 $ 2.64 $ 1.30 $ 4.17 $ 2.94 $ 3.12 $ (6.32) $ 11.55 $ 7.54
Earnings (loss) per Common Share Attributable to Cliffs Shareholders - Diluted:                      
Continuing operations (in usd per share) $ (11.58) $ 0.61 $ 1.79 $ 2.59 $ 1.18 $ 4.27 $ 2.80 $ 3.08 $ (6.57) $ 11.34 $ 7.32
Discontinued operations (in usd per share) $ 0.22 $ (0.02) $ 0.02 $ 0.04 $ 0.12 $ (0.12) $ 0.12 $ 0.03 $ 0.25 $ 0.14 $ 0.17
Earnings per common share attributable to Cliffs shareholders - Diluted (in usd per share) $ (11.36) $ 0.59 $ 1.81 $ 2.63 $ 1.30 $ 4.15 $ 2.92 $ 3.11 $ (6.32) $ 11.48 $ 7.49