Shareholders' Equity And Comprehensive Income (Changes In Comprehensive Income) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Beginning Balance			$ 3,845.9
Beginning Balance			(7.2)
Beginning Balance			3,838.7
Net income	589.5	297.4	1,420.6	635.4
Net income	151.8		170.1
Net income	741.3	297.4	1,590.7	635.4
Total comprehensive income	434.3	457.2	1,315.4	776.5
Share buyback			221.9
Ending Balance	5,723.4		5,723.4
Ending Balance	1,128.4		1,128.4
Ending Balance	6,851.8		6,851.8
Cliffs Shareholders' Equity [Member]
Beginning Balance			3,845.9
Net income			1,420.6
Other comprehensive income			(105.2)
Total comprehensive income			1,315.4
Share buyback			(221.9)
Equity offering			853.7
Purchase of additional noncontrolling interest			0.3
Stock and other incentive plans			8.8
Common stock dividends			(78.8)
Ending Balance	5,723.4		5,723.4
Non- Controlling Interest [Member]
Beginning Balance			(7.2)
Net income			170.1
Other comprehensive income			0.5
Total comprehensive income			170.6
Purchase of subsidiary shares from noncontrolling interest			4.5
Undistributed gains to noncontrolling interest			12.6
Capital contribution by noncontrolling interest to subsidiary			0.3
Acquisition of controlling interest			947.6
Ending Balance	1,128.4		1,128.4
Total Comprehensive Income [Member]
Beginning Balance			3,838.7
Net income			1,590.7
Other comprehensive income			(104.7)
Total comprehensive income			1,486.0
Share buyback			(221.9)
Equity offering			853.7
Purchase of additional noncontrolling interest			0.3
Stock and other incentive plans			8.8
Common stock dividends			(78.8)
Purchase of subsidiary shares from noncontrolling interest			4.5
Undistributed gains to noncontrolling interest			12.6
Capital contribution by noncontrolling interest to subsidiary			0.3
Acquisition of controlling interest			947.6
Ending Balance	$ 6,851.8		$ 6,851.8