Derivative Instruments And Hedging Activities (Narrative) (Detail)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended						3 Months Ended	9 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2012 U.S. Iron Ore, Eastern Canadian Iron Ore And Asia Pacific Iron Ore [Member] USD ($)	Sep. 30, 2012 U.S. Iron Ore, Eastern Canadian Iron Ore And Asia Pacific Iron Ore [Member] USD ($)	Sep. 30, 2012 Australian Dollar Foreign Exchange Contract Hedge Designation [Member] USD ($)	Dec. 31, 2011 Australian Dollar Foreign Exchange Contract Hedge Designation [Member] USD ($)	Sep. 30, 2011 Australian Dollar Foreign Exchange Contract Hedge Designation [Member] aud_per_usd	Sep. 30, 2012 Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] USD ($)	Dec. 31, 2011 Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] USD ($)	Sep. 30, 2012 Foreign Exchange Contract [Member] USD ($)	Sep. 30, 2012 Foreign Exchange Contract [Member] USD ($)	Sep. 30, 2011 Foreign Exchange Contract [Member] Consolidated Thompson [Member] USD ($)	Jan. 11, 2011 Foreign Exchange Contract [Member] Consolidated Thompson [Member] CAD	Sep. 30, 2012 Customer Contracts [Member] USD ($)	Dec. 31, 2011 Customer Contracts [Member] USD ($)	Sep. 30, 2012 Customer Contracts [Member] Product Revenue [Member] USD ($)	Sep. 30, 2011 Customer Contracts [Member] Product Revenue [Member] USD ($)	Sep. 30, 2012 Customer Contracts [Member] Product Revenue [Member] USD ($)	Sep. 30, 2011 Customer Contracts [Member] Product Revenue [Member] USD ($)	Sep. 30, 2012 Provisional Pricing Arrangements [Member] U.S. Iron Ore and Eastern Canadian Iron Ore [Member] Product Revenue [Member] USD ($)	Sep. 30, 2011 Provisional Pricing Arrangements [Member] U.S. Iron Ore and Eastern Canadian Iron Ore [Member] Product Revenue [Member] USD ($)	Sep. 30, 2012 Sonoma [Member] Asia Pacific Coal [Member] AUD	Dec. 31, 2011 Derivative Financial Instruments, Assets [Member] Provisional Pricing Arrangements [Member] U.S. Iron Ore and Eastern Canadian Iron Ore [Member] USD ($)	Dec. 31, 2011 Derivative Financial Instruments, Liabilities [Member] Provisional Pricing Arrangements [Member] U.S. Iron Ore and Eastern Canadian Iron Ore [Member] USD ($)	Dec. 31, 2011 Accounts Receivable [Member] Provisional Pricing Arrangements [Member] U.S. Iron Ore and Eastern Canadian Iron Ore [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] USD ($)	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Foreign Exchange Contract [Member] USD ($)	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] Foreign Exchange Contract [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Customer Contracts [Member] USD ($)	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] Customer Contracts [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Provisional Pricing Arrangements [Member] USD ($)	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] Provisional Pricing Arrangements [Member] USD ($)	Sep. 30, 2012 Not Designated as Hedging Instrument [Member] Liabilities Of Assets Held For Sale [Member] Foreign Exchange Contract [Member] USD ($)	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] Liabilities Of Assets Held For Sale [Member] Foreign Exchange Contract [Member] USD ($)
Derivative liability, fair value	$ 13,000,000		$ 13,000,000		$ 23,000,000																								$ 11,700,000	$ 19,500,000
Notional amounts of outstanding exchange rate contracts								420,000,000	400,000,000		645,700,000	0
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts								8,700,000			6,200,000
Ownership interest, percent																									45.00%
Proceeds from Divestiture of Interest in Joint Venture																									141,000,000
Amount of gain/(loss) recognized in income on derivative	0	(6,200,000)	300,000	100,500,000									1,100,000	1,100,000	93,100,000
Foreign Currency Spot Rate										1.04
Assets Held-for-sale, Current	156,600,000		156,600,000		159,900,000								1,100,000	1,100,000
Notional amount of foreign currency derivative instruments not designated as hedging instruments																4,700,000,000
Amount of gain/(loss) recognized in income on derivative						(10,300,000)	(10,300,000)												49,800,000	53,800,000	131,800,000	124,900,000	623,500,000	193,000,000
Derivative assets	78,300,000		78,300,000		82,100,000	1,400,000	1,400,000										54,500,000	72,900,000								1,200,000
Derivative liabilities						11,700,000	11,700,000																				19,500,000
Accounts receivable	$ 285,900,000		$ 285,900,000		$ 287,900,000																							$ 83,800,000