Acquisitions And Other Investments (Schedule Of Unaudited Consolidated Proforma Information) (Detail) (Consolidated Thompson [Member], USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011
Consolidated Thompson [Member]
Business Acquisition [Line Items]
REVENUES FROM PRODUCT SALES AND SERVICES	$ 2,065.0	$ 3,343.8
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 418.4	$ 810.1
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - BASIC	$ 3.01	$ 5.90
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - DILUTED	$ 2.99	$ 5.87