STOCK COMPENSATION PLANS (Share-based Compensation Expense Recorded For Continuing Operations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Selling, general and administrative expenses									$ 282.5	$ 248.3	$ 171.7
Reduction of operating income from continuing operations before income taxes and equity income (loss) from ventures									(501.8)	2,190.5	1,266.4
Income tax benefit									255.9	407.7	282.5
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS	(1,618.3)	85.1	258.0	375.8	185.4	601.2	409.1	423.4	(899.4)	1,619.1	1,019.9
Reduction of earnings per share attributable to Cliffs shareholders:
Basic (in usd per share)	$ (11.36)	$ 0.60	$ 1.81	$ 2.64	$ 1.30	$ 4.17	$ 2.94	$ 3.12	$ (6.32)	$ 11.55	$ 7.54
Diluted (in usd per share)	$ (11.36)	$ 0.59	$ 1.81	$ 2.63	$ 1.30	$ 4.15	$ 2.92	$ 3.11	$ (6.32)	$ 11.48	$ 7.49
Deferred Compensation, Share-based Payments [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Cost of goods sold and operating expenses									4.0	2.7	2.8
Selling, general and administrative expenses									16.6	13.2	12.7
Reduction of operating income from continuing operations before income taxes and equity income (loss) from ventures									20.6	15.9	15.5
Income tax benefit									(7.2)	(5.6)	(5.4)
NET INCOME (LOSS) ATTRIBUTABLE TO CLIFFS SHAREHOLDERS									$ 13.4	$ 10.3	$ 10.1
Reduction of earnings per share attributable to Cliffs shareholders:
Basic (in usd per share)									$ 0.09	$ 0.07	$ 0.07
Diluted (in usd per share)									$ 0.09	$ 0.07	$ 0.07