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	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011
Consolidated Thompson [Member]
Business Acquisition [Line Items]
REVENUES FROM PRODUCT SALES AND SERVICES	$ 2,089.1	$ 5,168.6
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	$ 607.3	$ 1,439.2
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - BASIC	$ 4.21	$ 10.31
EARNINGS PER COMMON SHARE ATTRIBUTABLE TO CLIFFS SHAREHOLDERS - DILUTED	$ 4.19	$ 10.26