Acquisitions and Other Investments (Consolidated Thompson Pro Forma Information) (Details) (Consolidated Thompson [Member], USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Consolidated Thompson [Member]
Total Revenues	$ 2,065.0	$ 1,219.9	$ 3,343.8	$ 1,947.4
Net Income	$ 417.0	$ 206.8	$ 808.7	$ 192.6
Earnings per common share attributable to Cliffs shareholders - Basic	$ 3.00	$ 1.53	$ 5.89	$ 1.42
Earnings per common share attributable to Cliffs shareholders - Diluted	$ 2.98	$ 1.52	$ 5.86	$ 1.42