Segment Reporting (Schedule Of Summary Of Assets, By Segment) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Total assets	$ 13,945.3	$ 7,778.2
U.S. Iron Ore [Member]
Total assets	1,772.2	1,537.1
Eastern Canadian Iron Ore [Member]
Total assets	7,712.6	629.6
North American Coal [Member]
Total assets	1,718.9	1,623.8
Asia Pacific Iron Ore [Member]
Total assets	1,343.1	1,195.3
Other Segment [Member]
Total assets	1,165.6	1,257.8
Total Segment Assets [Member]
Total assets	13,712.4	6,243.6
Corporate [Member]
Total assets	$ 232.9	$ 1,534.6