Annual report pursuant to Section 13 and 15(d)

Fair Value Of Financial Instruments (Narrative) (Details)

v2.4.0.6
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%