Fair Value Of Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 U.S. Iron Ore [Member]	Dec. 31, 2010 U.S. Iron Ore [Member]	Dec. 31, 2011 Empire [Member]	Dec. 31, 2010 Empire [Member]	Dec. 31, 2002 Empire [Member]	Dec. 31, 2001 Empire [Member]	Jun. 30, 2011 Significant Other Observable Inputs (Level 2) [Member] U.S. Iron Ore [Member]	Sep. 30, 2010 Significant Other Observable Inputs (Level 2) [Member] U.S. Iron Ore [Member]	Dec. 31, 2011 Significant Unobservable Inputs (Level 3) [Member]	Sep. 30, 2011 Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets And Liabilities Components [Line Items]
Derivative Asset, Fair Value	$ 165.9	$ 84.6							$ 20.0	$ 161.8	$ 83.8	$ 49.0
Interest rate on long term receivable	the higher of 9 percent or the prime rate plus 350 basis points
Percent ownership interest							46.70%	46.70%
Percent ownership interest after assuming all mine liabilities							79.00%
Maximum deferred portion of supplemental payments			22.3
Long Term Accounts Notes And Loans Receivable Net Noncurrent					26.5	32.8	120.0
Other non-current assets	191.2	187.9	22.3	22.3
Fair value of the receivable			$ 20.8	$ 19.5	$ 30.7	$ 38.9
Estimated credit-adjusted risk-free interest rate			4.50%		2.58%