Acquisitions And Other Investments (Schedule Of Unaudited Consolidated Proforma Information) (Details) (Consolidated Thompson [Member], USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2011
Consolidated Thompson [Member]
Business Acquisition [Line Items]
REVENUES FROM PRODUCT SALES AND SERVICES	$ 1,278.8
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 400.1
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - BASIC	$ 2.95
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - DILUTED	$ 2.94