Basis Of Presentation And Significant Accounting Policies (Investments In Unconsolidated Ventures) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Investment	$ 508.7	$ 504.8
Amapa [Member] | Investments In Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment	493.4	498.6
Interest Percentage	30.00%
Cockatoo [Member] | Other Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Investment	(16.7)	(15.0)
Interest Percentage	50.00%
Hibbing [Member] | Other Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Investment	(5.8)	(6.8)
Interest Percentage	23.00%
Other Equity Investees [Member] | Investments In Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment	$ 37.8	$ 28.0