ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Loss Contingencies [Line Items]
Total environmental and mine closure obligations	$ 265.1	$ 226.9
Payments related to environmental and mine closure liabilities	2.4	1.9
Environmental remediation	15.7	15.5
Mine Reclamation and Closing Liability, current and noncurrent	249.4	211.4
U.S. Iron Ore [Member]
Loss Contingencies [Line Items]
Mine closure obligation, number of mines (in number of facilities)	4
Eastern Canadian Iron Ore [Member]
Loss Contingencies [Line Items]
Mine closure obligation, number of mines (in number of facilities)	2
North American Coal [Member]
Loss Contingencies [Line Items]
Mine closure obligation, number of mines (in number of facilities)	6
LTV Steel Mining Company [Member]
Loss Contingencies [Line Items]
Mine closure obligation, number of mines (in number of facilities)	1
Asia Pacific Iron Ore [Member]
Loss Contingencies [Line Items]
Mine closure obligation, number of mines (in number of facilities)	1
Rio Tinto Mine Site [Member]
Loss Contingencies [Line Items]
Environmental remediation	11.5	10.0
Anticipated share of total settlement cost	12.0
Rio Tinto Working Group RTWG [Member] | Rio Tinto Mine Site [Member]
Loss Contingencies [Line Items]
Environmental remediation	$ 25.0