BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) T in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Facility T	Dec. 31, 2014 USD ($) T	Dec. 31, 2013 USD ($) shares	Jul. 10, 2012
Related Party Transaction [Line Items]
Deferred Tax Assets, Gross, Current		$ 23,700,000
Tangible Asset Impairment Charges		537,800,000
Inventory Valuation Reserves	$ 31,800,000	16,000,000
Trading Day Window Determining Number of Common Shares Issuable on Conversion	20
Depositary Share Interest in a Share of 7% Series A Mandatory Convertible Preferred Stock, Class A	0.025
Current Fiscal Year End Date	--12-31
Goodwill	$ 2,000,000	2,000,000	$ 74,500,000
Freight and venture partners' cost reimbursements	180,900,000	278,000,000	259,000,000
Allowance for Doubtful Accounts Receivable	7,100,000	0
Provision for Doubtful Accounts	$ 7,100,000		0.0
Derivative, Policy, Percentage of Amount Permitted to be Hedged	75.00%
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Twelve Months	40.00%
Derivative, Policy, Minimum Percentage of Amount Permitted to be Hedged for Derivatives with Periods of up to Fifteen Months	10.00%
Discontinued Operation, Tax Effect of Discontinued Operation	$ (6,000,000)	(1,216,000,000)	(184,500,000)
Asset Impairment Charges	3,300,000	635,500,000	14,300,000
Goodwill, Impairment Loss	0	73,500,000
Impairment of Intangible Assets (Excluding Goodwill)		13,800,000
Deferred Tax Liabilities, Gross, Current		4,000,000
Unamortized Debt Issuance Expense	$ 29,100,000	16,800,000
Amapa [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Other than Temporary Impairment			67,600,000
U.S. Iron Ore [Member]
Related Party Transaction [Line Items]
Number Of Mines | Facility	5
Goodwill	$ 2,000,000	2,000,000	2,000,000
Freight and venture partners' cost reimbursements	$ 157,300,000	$ 271,000,000	259,500,000
Quantity Of Finished Goods | T	1.3	1.4
Goodwill, Impairment Loss	$ 0	$ 0
Asia Pacific [Member]
Related Party Transaction [Line Items]
Freight and venture partners' cost reimbursements	$ 23,600,000	6,900,000
Asia Pacific Iron Ore [Member]
Related Party Transaction [Line Items]
Number Of Mines | Facility	1
Goodwill	$ 0	0	$ 72,500,000
Goodwill, Impairment Loss	$ 0	73,500,000
Sonoma [Member] | Asia Pacific Coal [Member]
Related Party Transaction [Line Items]
Ownership Interest				45.00%
Empire [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	79.00%
Bloom Lake [Member]
Related Party Transaction [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent			82.80%
Investments in Ventures [Member] | Amapa [Member]
Related Party Transaction [Line Items]
Ownership interest, equity method investment			30.00%
Low Volatile Metallurgical Coal Mines [Member] | North American Coal [Member]
Related Party Transaction [Line Items]
Number Of Mines | Facility	2
Take or Pay Contracts [Member]
Related Party Transaction [Line Items]
Deferred Revenue		29,300,000
Customer Supplemental Payments [Member] | U.S. Iron Ore [Member]
Related Party Transaction [Line Items]
Deferred Revenue	$ 89,900,000	102,800,000
Deferred Revenue, Current	12,800,000	12,800,000
Deferred Revenue, Noncurrent	$ 77,100,000	90,000,000
Wisco [Member]
Related Party Transaction [Line Items]
Increase in Subsidiary or Equity Method Investee Percentage Ownership Acquired			7.80%
Units Issued by CQIM to Bloom Lake for Acquisition of Noncontrolling Interest | shares			457,556
Subsidiary Or Equity Method Investee Percentage Ownership Acquired			17.152%
Increase/Decrease in Equity for WISCO/Bloom Lake through Acquisition of Noncontrolling Interest			$ 314,800,000
Bloom Lake [Member]
Related Party Transaction [Line Items]
Increase to Cliffs' Shareholders Equity through Acquisition of Noncontrolling Interest		314,800,000
Minimum [Member] | Other Machinery and Equipment [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Minimum [Member] | Electric Power Facilities [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Minimum [Member] | Land Improvements [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Minimum [Member] | Equipment [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Maximum [Member] | Other Machinery and Equipment [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	45 years
Maximum [Member] | Electric Power Facilities [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	45 years
Maximum [Member] | Land Improvements [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	45 years
Maximum [Member] | Equipment [Member]
Related Party Transaction [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Short-term intercompany loan [Member]
Related Party Transaction [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	$ 11,500,000	19,700,000	33,000,000
Transaction Gains and Losses Resulting from Remeasurement [Member]
Related Party Transaction [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	16,300,000	29,000,000	53,200,000
Cash and Cash Equivalents [Member]
Related Party Transaction [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	$ 1,500,000	$ 10,600,000	$ 20,400,000