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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Site Contingency [Line Items] |  |
Payments for Environmental Liabilities | $ 3.1 us-gaap_PaymentsForEnvironmentalLiabilities | $ 8.2 us-gaap_PaymentsForEnvironmentalLiabilities
Accrual for Environmental Loss Contingencies | 5.5 us-gaap_AccrualForEnvironmentalLossContingencies | 8.4 us-gaap_AccrualForEnvironmentalLossContingencies
Mine Reclamation and Closing Liability, current and noncurrent | 255.7 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent | 312.6 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
Total environmental and mine closure obligations | 261.2 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent | 321.0 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent
Current Fiscal Year End Date | --12-31 |
U.S. Iron Ore [Member] |  |
Site Contingency [Line Items] |  |
Mine closure obligation, number of mines (in number of facilities) | 5 clf_MineClosureObligationsNumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Eastern Canadian Iron Ore [Member] |  |
Site Contingency [Line Items] |  |
Mine closure obligation, number of mines (in number of facilities) | 2 clf_MineClosureObligationsNumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Asia Pacific Iron Ore [Member] |  |
Site Contingency [Line Items] |  |
Mine closure obligation, number of mines (in number of facilities) | 1 clf_MineClosureObligationsNumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
North American Coal [Member] |  |
Site Contingency [Line Items] |  |
Mine closure obligation, number of mines (in number of facilities) | 2 clf_MineClosureObligationsNumberOfMines / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
LTV Steel Mining Company [Member] |  |
Site Contingency [Line Items] |  |
Mine closure obligation, number of mines (in number of facilities) | 1 clf_MineClosureObligationsNumberOfMines / clf_MineClosureAxis = clf_LTVSteelMiningCompanyMember |
Rio Tinto Mine Site [Member] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies | 2.5 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = clf_RioTintoMineSiteMember | 5.3 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = clf_RioTintoMineSiteMember
Loss Contingency, Range of Possible Loss, Maximum |  | 12.2 us-gaap_LossContingencyRangeOfPossibleLossMaximum / us-gaap_EnvironmentalRemediationSiteAxis = clf_RioTintoMineSiteMember
Rio Tinto Mine Site [Member] | Rio Tinto Working Group RTWG [Member] |  |
Site Contingency [Line Items] |  |
Accrual for Environmental Loss Contingencies |  | $ 29.0 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_CounterpartyNameAxis = clf_RioTintoWorkingGroupRtwgMember / us-gaap_EnvironmentalRemediationSiteAxis = clf_RioTintoMineSiteMember

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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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