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