Goodwill and Other Intangible Assets and Liabilities (Changes in the Carrying Amount of Goodwill Allocated by Reporting Unit) (Details) (USD $) In Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2011		Dec. 31, 2010
Goodwill, beginning balance		$ 196.5	[1]	$ 74.6	[1]
Arising in business combinations	1,026.8	1,026.8		111.9	[1]
Impact of foreign currency translation		4.5		10.0	[1]
Other		(0.5)
Goodwill, ending balance	1,227.3	1,227.3		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		2.0	[1]
Eastern Canadian Iron Ore [Member]
Goodwill, beginning balance		3.1	[1]
Arising in business combinations		1,026.8		3.1	[1]
Other		(0.4)
Goodwill, ending balance	1,029.5	1,029.5		3.1	[1]
Asia Pacific Iron Ore [Member]
Goodwill, beginning balance		82.6	[1]	72.6	[1]
Impact of foreign currency translation		4.5		10.0	[1]
Goodwill, ending balance	87.1	87.1		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	27.8		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		$ 80.9	[1]

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