Quarterly report pursuant to Section 13 or 15(d)

Fair Value Of Financial Instruments (Narrative) (Detail)

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Fair Value Of Financial Instruments (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended 6 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Jun. 30, 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]
Jun. 30, 2012
U.S. Iron Ore [Member]
Dec. 31, 2011
U.S. Iron Ore [Member]
Fair Value, Assets And Liabilities Components [Line Items]                      
Transfer of derivative assets from level 3 to level 2 $ 20.0   $ 20.0                
Maximum deferred portion of supplemental payments                   22.3  
Interest rate on long term receivables   The higher of 9 percent or the prime rate plus 350 basis points                  
Other non-current assets   212.2   191.2           11.1 22.3
Fair value of the receivable         26.2 30.7       10.5 20.8
Estimated credit-adjusted risk-free interest rate         2.12%         3.29%  
Percent ownership interest               46.70% 79.00%    
Long term accounts notes and loans receivable net noncurrent         $ 23.0 $ 26.5 $ 120.0