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, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
LOSS FROM DISCONTINUED OPERATIONS, net of tax $ (19.9) $ (892.1) $ (8,368.0)
Income (Loss) from Discontinued Operations, net of tax, including portion attributable to Noncontrolling interest, Canadian Entities and North American Coal     (8,368.0)
Short-term assets of discontinued operations   14.9  
Short-term liabilities of discontinued operations 6.0 6.9  
Depreciation, depletion and amortization 115.4 134.0 504.0
Payments to Acquire Property, Plant, and Equipment 69.1 80.8 284.1
Impairment of goodwill and other long-lived assets 0.0 76.6 9,029.9
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 (2.4) (152.4) (1,134.5)
Short-term assets of discontinued operations   14.9  
Short-term liabilities of discontinued operations 6.0 6.9  
Depreciation, depletion and amortization   3.2 106.9
Payments to Acquire Property, Plant, and Equipment   15.9 29.9
Impairment of goodwill and other long-lived assets   73.4 857.5
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 (17.5) (638.7) (6,952.9)
Short-term assets of discontinued operations   0.0  
Short-term liabilities of discontinued operations 0.0 0.0  
Depreciation, depletion and amortization   0.0 135.6
Payments to Acquire Property, Plant, and Equipment   0.0 190.3
Impairment of goodwill and other long-lived assets   0.0 7,269.2
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 (101.0) (280.6)
Short-term assets of discontinued operations   0.0  
Short-term liabilities of discontinued operations 0.0 0.0  
Depreciation, depletion and amortization   0.0 0.5
Payments to Acquire Property, Plant, and Equipment   0.0 0.0
Impairment of goodwill and other long-lived assets   0.0 267.6
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 (17.5) (739.7) (7,233.5)
Short-term assets of discontinued operations   0.0  
Short-term liabilities of discontinued operations $ 0.0 0.0  
Depreciation, depletion and amortization   0.0 136.1
Payments to Acquire Property, Plant, and Equipment   0.0 190.3
Impairment of goodwill and other long-lived assets   0.0 7,536.8
Discontinued Operations [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Depreciation, depletion and amortization   3.2 243.0
Payments to Acquire Property, Plant, and Equipment   15.9 220.2
Impairment of goodwill and other long-lived assets   $ 73.4 $ 8,394.3