Shareholders' Equity And Comprehensive Income (Components Of Comprehensive Income) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Shareholders' Equity And Comprehensive Income [Abstract]
Net income attributable to Cliffs shareholders	$ 589.5	$ 297.4	$ 1,420.6	$ 635.4
Unrealized net gain (loss) on marketable securities - net of tax	(11.6)	14.5	(30.8)	0.3
Foreign currency translation	(132.2)	145.8	(74.8)	101.5
Amortization of net periodic benefit cost - net of tax	5.3	(0.5)	13.9	42.5
Reclassification of net gains on derivative financial instruments into net income	(1.5)		(3.2)	(3.2)
Unrealized loss on derivative financial instruments	(15.2)		(10.3)
Total other comprehensive income (loss)	(155.2)	159.8	(105.2)	141.1
Total comprehensive income	$ 434.3	$ 457.2	$ 1,315.4	$ 776.5