Annual report pursuant to Section 13 and 15(d)

DISCONTINUED OPERATIONS Schedule of Disposal Groups, including Discontinued Operations, Income Statement, Balance Sheet and Cash Flows (Details)

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DISCONTINUED OPERATIONS Schedule of Disposal Groups, including Discontinued Operations, Income Statement, Balance Sheet and Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax $ (18.7) $ (19.9) $ (892.1)
Income (Loss) from Discontinued Operations, net of tax, including portion attributable to Noncontrolling interest, Canadian Entities and North American Coal     (892.1)
Short-term liabilities of discontinued operations 3.2 6.0  
Depreciation, depletion and amortization 87.7 115.4 134.0
Purchase of property, plant and equipment 151.7 69.1 80.8
Impairment of goodwill and other long-lived assets 0.0 0.0 76.6
North American Coal [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax 2.6 (2.4) (152.4)
Short-term liabilities of discontinued operations 3.2 6.0  
Depreciation, depletion and amortization     3.2
Purchase of property, plant and equipment     15.9
Impairment of goodwill and other long-lived assets     73.4
Eastern Canadian Iron Ore [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax (21.3) (17.5) (638.7)
Short-term liabilities of discontinued operations 0.0 0.0  
Depreciation, depletion and amortization     0.0
Purchase of property, plant and equipment     0.0
Impairment of goodwill and other long-lived assets     0.0
Other Canadian Operations [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax 0.0 0.0 (101.0)
Short-term liabilities of discontinued operations 0.0 0.0  
Depreciation, depletion and amortization     0.0
Purchase of property, plant and equipment     0.0
Impairment of goodwill and other long-lived assets     0.0
Canadian Entities [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax (21.3) (17.5) (739.7)
Short-term liabilities of discontinued operations $ 0.0 $ 0.0  
Depreciation, depletion and amortization     0.0
Purchase of property, plant and equipment     0.0
Impairment of goodwill and other long-lived assets     0.0
Discontinued Operations [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Depreciation, depletion and amortization     3.2
Purchase of property, plant and equipment     15.9
Impairment of goodwill and other long-lived assets     $ 73.4