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, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
INCOME FROM DISCONTINUED OPERATIONS, net of tax	$ 88.2	$ 2.5	$ 76.7
Current assets of discontinued operations	12.4	118.5		$ 118.5
Non-current assets of discontinued operations	0.0	20.3		20.3
Current liabilities of discontinued operations	6.7	75.0		75.0
Non-current liabilities of discontinued operations	8.3	52.2		$ 52.2
Net cash provided (used) by operating activities	478.5	338.1	303.0
Net cash provided (used) by investing activities	(273.1)	(156.0)	(57.9)
Asia Pacific Iron Ore [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
INCOME FROM DISCONTINUED OPERATIONS, net of tax	118.3	21.2	96.6
Current assets of discontinued operations	12.4	118.5
Non-current assets of discontinued operations	0.0	20.3
Current liabilities of discontinued operations	3.8	71.8
Non-current liabilities of discontinued operations	8.3	52.2
North American Coal [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
INCOME FROM DISCONTINUED OPERATIONS, net of tax	(3.6)	2.6	(2.4)
Current assets of discontinued operations	0.0	0.0
Non-current assets of discontinued operations	0.0	0.0
Current liabilities of discontinued operations	2.9	3.2
Non-current liabilities of discontinued operations	0.0	0.0
Canadian Entities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
INCOME FROM DISCONTINUED OPERATIONS, net of tax	(26.5)	(21.3)	(17.5)
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided (used) by operating activities	(95.9)	79.6	99.8
Net cash provided (used) by investing activities	19.8	(8.4)	10.0
Discontinued Operations [Member] | Asia Pacific Iron Ore [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided (used) by operating activities	(81.3)	79.6	99.8
Net cash provided (used) by investing activities	19.8	(2.8)	(0.4)
Discontinued Operations [Member] | North American Coal [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided (used) by investing activities	0.0	2.1	3.6
Discontinued Operations [Member] | Canadian Entities [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net cash provided (used) by operating activities	(14.6)	0.0	0.0
Net cash provided (used) by investing activities	$ 0.0	$ (7.7)	$ 6.8