Goodwill And Other Intangible Assets And Liabilities (Schedule Of Goodwill) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011
Goodwill [Roll Forward]
Beginning Balance	$ 1,152.1		$ 196.5
Arising in business combinations	13.8		983.4
Impairment			(27.8)
Impact of foreign currency translation	1.3		0.4
Other			(0.4)
Ending Balance	1,167.2		1,152.1
U.S. Iron Ore [Member]
Goodwill [Roll Forward]
Beginning Balance	2.0	[1]	2.0	[1]
Arising in business combinations				[1]
Impairment				[1]
Impact of foreign currency translation				[1]
Other				[1]
Ending Balance	2.0		2.0	[1]
Eastern Canadian Iron Ore [Member]
Goodwill [Roll Forward]
Beginning Balance	986.2	[1]	3.1	[1]
Arising in business combinations	13.8		983.5	[1]
Impairment				[1]
Impact of foreign currency translation				[1]
Other			(0.4)	[1]
Ending Balance	1,000.0		986.2	[1]
Asia Pacific Iron Ore [Member]
Goodwill [Roll Forward]
Beginning Balance	83.0	[1]	82.6	[1]
Arising in business combinations				[1]
Impairment				[1]
Impact of foreign currency translation	1.3		0.4	[1]
Other				[1]
Ending Balance	84.3		83.0	[1]
North American Coal [Member]
Goodwill [Roll Forward]
Beginning Balance			27.9	[1]
Arising in business combinations			(0.1)	[1]
Impairment			(27.8)	[1]
Impact of foreign currency translation				[1]
Other				[1]
Ending Balance
Other Segment [Member]
Goodwill [Roll Forward]
Beginning Balance	80.9	[1]	80.9	[1]
Arising in business combinations				[1]
Impairment				[1]
Impact of foreign currency translation				[1]
Other				[1]
Ending Balance	$ 80.9		$ 80.9	[1]

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