FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended									12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Empire [Member]	Dec. 31, 2011 Empire [Member]	Dec. 31, 2002 Empire [Member]	Dec. 31, 2002 Prior [Member] Empire [Member]	Dec. 31, 2002 Current [Member] Empire [Member]	Dec. 31, 2012 Other Current Assets [Member] Empire [Member]	Dec. 31, 2011 Other Current Assets [Member] Empire [Member]	Dec. 31, 2012 U.S. Iron Ore [Member]	Dec. 31, 2011 U.S. Iron Ore [Member]	Dec. 31, 2012 Amapa [Member]	Dec. 31, 2012 Wabush [Member]	Dec. 31, 2012 Wabush [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Transfer of derivative assets from level 3 to level 2		$ 20.0	$ 0	$ 20.0
Transfer of derivative assets from level 2 to level 3	49.0		0	49.0
Management Estimate of 62% Fe			62.00%
Maximum deferred portion of supplemental payments												22.3
Supply agreement, customer receivable, stated interest rate			9.00%
Supply agreement, customer receivable, description of variable rate basis			prime rate
Other current assets	166.3		216.2	166.3									22.3
Supply agreement, customer receivable, basis spread on variable rate			3.50%
Other non-current assets	191.2		193.6	191.2									22.3
Fair value of the receivable					21.3	30.7						21.3	20.8
Estimated credit-adjusted risk-free interest rate					2.85%	2.58%						2.81%	4.50%
Interest Percentage								46.70%
Equity Method Investments, Fair Value Disclosure														72.5
Percent ownership interest after transaction									79.00%
Long term accounts notes and loans receivable net noncurrent					19.3	26.5	120.0			10.0	10.0
Goodwill impairment			1,000.0	27.8											2.7	2.7
Impairment of long-lived assets																$ 49.9