Basis Of Presentation And Significant Accounting Policies (Investments In Unconsolidated Ventures) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Investment	$ 488.3	$ 504.8
Amapa [Member] \| Investments In Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Percent ownership interest	30.00%
Investment	479.2	498.6
Cockatoo [Member] \| Other Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Percent ownership interest	50.00%
Investment	(28.7)	(15.0)
Hibbing [Member] \| Other Liabilities [Member]
Schedule of Equity Method Investments [Line Items]
Investment	0.8	(6.8)
Other Equity Investees [Member] \| Investments In Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investment	$ 37.0	$ 28.0

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph a (1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant