Basis of Presentation and Significant Accounting Policies (Investments in Unconsolidated Venture) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Investment	$ 507.6	$ 509.0
Other Liabilities [Member] | Hibbing [Member]
Interest Percentage	23.00%
Investment	(1.8)	(5.8)
Investments in Ventures [Member] | Amapa [Member]
Interest Percentage	30.00%
Investment	476.7	461.3
Investments in Ventures [Member] | AusQuest [Member]
Interest Percentage	30.00%
Investment	7.3	24.1
Investments in Ventures [Member] | Cockatoo [Member]
Interest Percentage	50.00%
Investment	6.1	10.5
Investments in Ventures [Member] | Other Equity Investees [Member]
Investment	$ 19.3	$ 18.9