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