Shareholders' Equity (Schedule Of Stockholders' Equity) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Balance			$ 7,039.7	$ 3,838.7
Net income	274.3	427.4	665.7	850.8
Other comprehensive income	(17.5)	26.4	5.7	51.0
Total comprehensive income			671.4	901.8
Equity offering				853.7
Purchase of additional noncontrolling interest				0.3
Stock and other incentive plans			2.3	3.1
Common stock dividends			(128.8)	(38.0)
Purchase of subsidiary shares from noncontrolling interest				4.5
Undistributed gains to noncontrolling interest			8.6	9.6
Capital contribution by noncontrolling interest to subsidiary			22.3	0.2
Acquisition of controlling interest			(8.0)	947.6
Balance	7,607.5	6,521.5	7,607.5	6,521.5
Cliffs Shareholders' Equity [Member]
Balance			5,785.0	3,845.9
Net income			633.8	832.5
Other comprehensive income			2.7	50.1
Total comprehensive income			636.5	882.6
Equity offering				853.7
Purchase of additional noncontrolling interest				0.3
Stock and other incentive plans			2.3	3.1
Common stock dividends			(128.8)	(38.0)
Balance	6,295.0	5,547.6	6,295.0	5,547.6
Non-controlling Interest [Member]
Balance			1,254.7	(7.2)
Net income			31.9	18.3
Other comprehensive income			3.0	0.9
Total comprehensive income			34.9	19.2
Purchase of subsidiary shares from noncontrolling interest				4.5
Undistributed gains to noncontrolling interest			8.6	9.6
Capital contribution by noncontrolling interest to subsidiary			22.3	0.2
Acquisition of controlling interest			(8.0)	947.6
Balance	$ 1,312.5	$ 973.9	$ 1,312.5	$ 973.9
Bloom Lake [Member]
Percentage Of Ownership Interests	75.00%	75.00%	75.00%	75.00%
Tilden [Member]
Percentage Of Ownership Interests	85.00%	85.00%	85.00%	85.00%
Empire [Member]
Percentage Of Ownership Interests	79.00%	79.00%	79.00%	79.00%