Shareholders' Equity (Schedule Of Stockholders' Equity) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Balance			$ 5,785.0
Balance			1,254.7
Balance			7,039.7	3,838.7
Net income	85.1	601.2	719.0	1,433.6
Net income	6.7	(151.8)	(25.2)	(170.1)
Net income	78.4	753.0	744.2	1,603.7
Other comprehensive income	123.9	446.0	760.6	1,328.4
Other comprehensive income	40.3	(155.6)	46.1	(104.7)
Total comprehensive income	(1.5)	0.4	(4.5)	(0.5)
Total comprehensive income			790.3	1,499.0
Share buyback		(222.0)		(221.9)
Equity offering				853.7
Purchase of additional noncontrolling interest				0.3
Stock and other incentive plans			9.0	8.8
Common stock dividends			(217.8)	(78.8)
Purchase of subsidiary shares from noncontrolling interest				4.5
Undistributed gains to noncontrolling interest			11.3	12.6
Capital contribution by noncontrolling interest to subsidiary			64.7	0.3
Acquisition of controlling interest			(8.0)	947.6
Balance	6,336.8		6,336.8
Balance	1,352.4		1,352.4
Balance	7,689.2	6,864.8	7,689.2	6,864.8
Cliffs Shareholders' Equity [Member]
Balance			5,785.0	3,845.9
Net income			719.0	1,433.6
Other comprehensive income			41.6	(105.2)
Total comprehensive income			760.6	1,328.4
Share buyback				(221.9)
Equity offering				853.7
Purchase of additional noncontrolling interest				0.3
Stock and other incentive plans			9.0	8.8
Common stock dividends			(217.8)	(78.8)
Purchase of subsidiary shares from noncontrolling interest				0
Undistributed gains to noncontrolling interest			0	0
Capital contribution by noncontrolling interest to subsidiary			0	0
Acquisition of controlling interest			0	0
Balance	6,336.8	5,736.4	6,336.8	5,736.4
Non-controlling Interest [Member]
Balance			1,254.7	(7.2)
Net income			25.2	170.1
Other comprehensive income			4.5	0.5
Total comprehensive income			29.7	170.6
Share buyback				0
Equity offering				0
Purchase of additional noncontrolling interest				0
Stock and other incentive plans			0	0
Common stock dividends			0	0
Purchase of subsidiary shares from noncontrolling interest				4.5
Undistributed gains to noncontrolling interest			11.3	12.6
Capital contribution by noncontrolling interest to subsidiary			64.7	0.3
Acquisition of controlling interest			(8.0)	947.6
Balance	$ 1,352.4	$ 1,128.4	$ 1,352.4	$ 1,128.4
Bloom Lake [Member]
Ownership interest, percent	75.00%	75.00%	75.00%	75.00%
Tilden [Member]
Ownership interest, percent	85.00%	85.00%	85.00%	85.00%
Empire [Member]
Ownership interest, percent	79.00%	79.00%	79.00%	79.00%