Goodwill And Other Intangible Assets And Liabilities (Schedule Of Goodwill) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended							6 Months Ended		12 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2012 U.S. Iron Ore [Member]	Dec. 31, 2011 U.S. Iron Ore [Member]		Dec. 31, 2010 U.S. Iron Ore [Member]		Jun. 30, 2012 Eastern Canadian Iron Ore [Member]		Dec. 31, 2011 Eastern Canadian Iron Ore [Member]		Dec. 31, 2011 North American Coal [Member]		Jun. 30, 2012 Asia Pacific Iron Ore [Member]		Dec. 31, 2011 Asia Pacific Iron Ore [Member]		Jun. 30, 2012 Other [Member]	Dec. 31, 2011 Other [Member]		Dec. 31, 2010 Other [Member]
Region Reporting Information, by Region [Line Items]
Beginning Balance	$ 1,152.1	[1]	$ 196.5	[1]	$ 2.0	$ 2.0	[1]	$ 2.0	[1]	$ 986.2	[1]	$ 3.1	[1]	$ 27.9	[1]	$ 83.0	[1]	$ 82.6	[1]	$ 80.9	$ 80.9	[1]	$ 80.9	[1]
Arising in business combinations	13.8		983.4	[1]						13.8		983.5	[1]	(0.1)	[1]
Impairment			(27.8)	[1]										(27.8)	[1]
Impact of foreign currency translation	0.2		0.4	[1]												0.2		0.4	[1]
Other			(0.4)	[1]								(0.4)	[1]
Ending Balance	$ 1,166.1		$ 1,152.1	[1]	$ 2.0	$ 2.0	[1]	$ 2.0	[1]	$ 1,000.0		$ 986.2	[1]			$ 83.2		$ 83.0	[1]	$ 80.9	$ 80.9	[1]	$ 80.9	[1]

[1] Represents a 12-Month rollforward of our goodwill by reportable unit at December 31, 2011.