Goodwill And Other Intangible Assets And Liabilities (Changes In The Carrying Amount Of Goodwill Allocated By Reporting Unit) (Details) (USD $) In Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Goodwill, beginning balance	$ 196.5	[1]	$ 74.6	[1]
Arising in business combinations	1,045.8		111.9	[1]
Impact of foreign currency translation	(4.1)		10.0	[1]
Other	(0.5)
Goodwill, ending balance	1,237.7		196.5	[1]
U.S. Iron Ore [Member]
Goodwill, beginning balance	2.0	[1]	2.0	[1]
Arising in business combinations				[1]
Impact of foreign currency translation				[1]
Other				[1]
Goodwill, ending balance	2.0		2.0	[1]
Eastern Canadian Iron Ore [Member]
Goodwill, beginning balance	3.1	[1]
Arising in business combinations	1,045.8		3.1	[1]
Other	(0.4)
Goodwill, ending balance	1,048.5		3.1	[1]
Asia Pacific Iron Ore [Member]
Goodwill, beginning balance	82.6	[1]	72.6	[1]
Impact of foreign currency translation	(4.1)		10.0	[1]
Goodwill, ending balance	78.5		82.6	[1]
North American Coal [Member]
Goodwill, beginning balance	27.9	[1]
Arising in business combinations			27.9	[1]
Other	(0.1)
Goodwill, ending balance	27.8		27.9	[1]
Other Goodwill [Member]
Goodwill, beginning balance	80.9	[1]
Arising in business combinations			80.9	[1]
Impact of foreign currency translation
Other
Goodwill, ending balance	$ 80.9		$ 80.9	[1]

[1] Represents a 12-month rollforward of our goodwill by reportable unit at December 31, 2010.