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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
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12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Dec. 31, 2014
USD ($)
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Dec. 31, 2013
USD ($)
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Dec. 31, 2012
USD ($)
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Dec. 31, 2014
Northern Lands [Member]
USD ($)
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Dec. 31, 2013
Amapa [Member]
USD ($)
|
Dec. 31, 2012
Amapa [Member]
USD ($)
|
Dec. 31, 2014
U.S. Iron Ore [Member]
USD ($)
Facility
T
|
Dec. 31, 2013
U.S. Iron Ore [Member]
USD ($)
T
|
Dec. 31, 2012
U.S. Iron Ore [Member]
USD ($)
|
Dec. 31, 2014
Asia Pacific [Member]
USD ($)
|
Dec. 31, 2014
Asia Pacific Iron Ore [Member]
USD ($)
Facility
|
Dec. 31, 2013
Asia Pacific Iron Ore [Member]
USD ($)
|
Dec. 31, 2012
Asia Pacific Iron Ore [Member]
USD ($)
|
Dec. 31, 2013
Asia Pacific Iron Ore [Member]
Cockatoo [Member]
USD ($)
|
Dec. 31, 2013
Asia Pacific Iron Ore [Member]
Cockatoo [Member]
AUD
|
Dec. 31, 2013
Other Segment [Member]
USD ($)
|
Dec. 31, 2014
Other Segment [Member]
USD ($)
|
Dec. 31, 2012
Other Segment [Member]
USD ($)
|
Dec. 31, 2014
North American Coal [Member]
USD ($)
|
Dec. 31, 2013
North American Coal [Member]
USD ($)
|
Dec. 31, 2012
North American Coal [Member]
USD ($)
|
Dec. 31, 2014
Eastern Canadian Iron Ore [Member]
USD ($)
Facility
|
Dec. 31, 2013
Eastern Canadian Iron Ore [Member]
USD ($)
|
Dec. 31, 2012
Eastern Canadian Iron Ore [Member]
USD ($)
|
Dec. 31, 2013
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]
USD ($)
|
Dec. 31, 2013
CQIM Reporting Unit [Member]
USD ($)
|
Dec. 31, 2014
Wabush [Member]
USD ($)
|
Dec. 31, 2013
Wabush [Member]
USD ($)
|
Dec. 31, 2013
Sonoma [Member]
Asia Pacific Coal [Member]
AUD
|
Jul. 10, 2012
Sonoma [Member]
Asia Pacific Coal [Member]
|
Dec. 31, 2014
Empire [Member]
|
Dec. 31, 2014
Big Daddy Chromite Deposit [Member]
|
Dec. 31, 2014
CLCC [Member]
|
Dec. 31, 2014
Wabush [Member]
|
Dec. 31, 2013
Bloom Lake [Member]
USD ($)
|
Dec. 31, 2013
Bloom Lake [Member]
USD ($)
|
Dec. 31, 2012
Bloom Lake [Member]
USD ($)
|
Dec. 31, 2011
Bloom Lake [Member]
USD ($)
|
Dec. 31, 2014
Bloom Lake [Member]
|
Dec. 31, 2013
Investments in Ventures [Member]
Amapa [Member]
|
Dec. 31, 2014
Low Volatile Metallurgical Coal Mines [Member]
North American Coal [Member]
Facility
|
Dec. 31, 2014
High volatile metallurgical coal mines [Member]
North American Coal [Member]
Facility
|
Dec. 31, 2014
Thermal Coal Mines [Member]
North American Coal [Member]
Facility
|
Dec. 31, 2014
Take or Pay Contracts [Member]
USD ($)
|
Dec. 31, 2013
Take or Pay Contracts [Member]
USD ($)
|
Dec. 31, 2014
Customer Supplemental Payments [Member]
U.S. Iron Ore [Member]
USD ($)
|
Dec. 31, 2013
Customer Supplemental Payments [Member]
U.S. Iron Ore [Member]
USD ($)
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Dec. 31, 2014
Wisco [Member]
|
Dec. 31, 2013
Wisco [Member]
USD ($)
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Dec. 31, 2013
Bloom Lake [Member]
USD ($)
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Dec. 31, 2014
Minimum [Member]
Other Machinery and Equipment [Member]
|
Dec. 31, 2014
Minimum [Member]
Electric Power Facilities [Member]
|
Dec. 31, 2014
Minimum [Member]
Land Improvements [Member]
|
Dec. 31, 2014
Minimum [Member]
Equipment [Member]
|
Dec. 31, 2014
Maximum [Member]
Other Machinery and Equipment [Member]
|
Dec. 31, 2014
Maximum [Member]
Electric Power Facilities [Member]
|
Dec. 31, 2014
Maximum [Member]
Land Improvements [Member]
|
Dec. 31, 2014
Maximum [Member]
Equipment [Member]
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| Related Party Transaction [Line Items] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Tangible Asset Impairment Charges | $ 8,839,000,000 | $ 145,100,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Impairment of Long-Lived Assets Held-for-use | 140,100,000 | 49,900,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Inventory Valuation Reserves | 68,200,000 | 63,400,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Number Of Mines | 5 | 1 | 2 | 2 | 2 | 1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Deferred Revenue | 29,300,000 | 13,500,000 | 102,800,000 | 115,600,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ownership Interest | 45.00% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Increase in Subsidiary or Equity Method Investee Percentage Ownership Acquired | 7.80% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Units Issued by CQIM to Bloom Lake for Acquisition of Noncontrolling Interest | 457,556 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ownership interest, equity method investment | 30.00% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Total percentage of Amapa sold | 100.00% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Equity Method Investment, Other than Temporary Impairment | 0 | 0 | (365,400,000) | 21,500,000 | 67,600,000 | (365,400,000) | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| Proceeds from sale of economic interest (in AUD) | 4.00 | 141,000,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Estimated rehabilitation costs | 18,600,000 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Goodwill | 2,000,000 | 74,500,000 | 167,400,000 | 2,000,000 | 2,000,000 | 2,000,000 | 0 | 72,500,000 | 84,500,000 | 0 | 0 | 80,900,000 | 0 | 0 | 0 | |||||||||||||||||||||||||||||||||||||||||||
| Reimbursement Revenue | 392,900,000 | 344,800,000 | 351,800,000 | 271,000,000 | 259,500,000 | 250,800,000 | 6,900,000 | 115,000,000 | 85,800,000 | 101,000,000 | ||||||||||||||||||||||||||||||||||||||||||||||||
| Allowance for Doubtful Accounts Receivable | 8,100,000 | 8,100,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Provision for Doubtful Accounts | 0 | 0 | 9,000,000 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Quantity Of Finished Goods | 1,400,000 | 1,200,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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% | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Deferred Revenue, Current | 12,800,000 | 12,800,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Deferred Revenue, Noncurrent | 90,000,000 | 102,800,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Asset Impairment Charges | 9,029,900,000 | 250,800,000 | 1,049,900,000 | 154,600,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Noncontrolling Interest, Ownership Percentage by Parent | 79.00% | 70.00% | 100.00% | 100.00% | 75.00% | 82.80% | ||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Increase to Cliffs' Shareholders Equity through Acquisition of Noncontrolling Interest | 314,800,000 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Immaterial Error Adjustment Related to Bloom Lake Purchase Price Acquisition | 45,100,000 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Change in Basic EPS for Immaterial Error Adjustment Related to Bloom Lake Purchase Price Acquisition | $ 0.29 | $ 0.30 | $ 0 | $ 0.07 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Change in Diluted EPS for Immaterial Error Adjustment Related to Bloom Lake Purchase Price Acquisition | $ 0.26 | $ 0.25 | $ 0.07 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Goodwill, Impairment Loss | 73,500,000 | 80,900,000 | 0 | 0 | 73,500,000 | 0 | (80,900,000) | 0 | 0 | 80,900,000 | 997,300,000 | |||||||||||||||||||||||||||||||||||||||||||||||
| Property, Plant and Equipment, Useful Life | 10 years | 10 years | 20 years | 3 years | 45 years | 45 years | 45 years | 20 years | ||||||||||||||||||||||||||||||||||||||||||||||||||
| Impairment of Intangible Assets (Excluding Goodwill) | $ 15,500,000 | $ 14,500,000 | $ 9,500,000 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||