Annual report pursuant to Section 13 and 15(d)

Derivative Instruments And Hedging Activities (Derivatives Not Designated As Hedging Instruments Disclosure) (Details)

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Derivative Instruments And Hedging Activities (Derivatives Not Designated As Hedging Instruments Disclosure) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 1,090.0 $ 1,131.8 $ 191.6
Foreign Exchange Contracts [Member] | Product Revenues [Member]
      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 1.0 11.1 5.4
Foreign Exchange Contracts [Member] | Other Income (Expense) [Member]
      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 101.9 39.8 85.7
Customer Supply Agreements [Member] | Product Revenues [Member]
      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 178.0 120.2 22.2
Provisional Pricing Arrangements [Member] | Product Revenues [Member]
      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net 809.1 960.7 (28.2)
United Taconite [Member] | Product Revenues [Member]
      
Derivative [Line Items]      
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net     $ 106.5