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CAPITAL STOCK (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 2 Months Ended | 3 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Feb. 21, 2013 | Feb. 11, 2013 | Nov. 19, 2014 | Nov. 03, 2014 | Aug. 01, 2014 | Feb. 03, 2014 | Nov. 01, 2013 | Aug. 01, 2013 | Mar. 20, 2013 | Sep. 02, 2014 | Jun. 03, 2014 | Mar. 03, 2014 | Dec. 02, 2013 | Sep. 03, 2013 | Jun. 03, 2013 | Mar. 01, 2013
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Depositary Share Interest in a Share of 7% Series A Mandatory Convertible Preferred Stock, Class A |  |  |  |  | 0.025 clf_DepositaryShareInterestInShareOf7SeriesMandatoryConvertiblePreferredStockClassA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Par or Stated Value Per Share | $ 0.125 us-gaap_CommonStockParOrStatedValuePerShare |  |  |  | $ 0.125 us-gaap_CommonStockParOrStatedValuePerShare | $ 0.125 us-gaap_CommonStockParOrStatedValuePerShare |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trading Day Window Determining Number of Common Shares Issuable on Conversion |  |  |  |  | 20 clf_TradingDayWindowDeterminingNumberOfCommonSharesIssuableOnConversion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from issuance of Series A, Mandatory Convertible Preferred Stock, Class A |  |  |  |  | $ 0 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock | $ 709.4 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock | $ 0 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends, Preferred Stock | 12.8 us-gaap_DividendsPreferredStock | 12.8 us-gaap_DividendsPreferredStock | 12.8 us-gaap_DividendsPreferredStock | 12.8 us-gaap_DividendsPreferredStock | 51.2 us-gaap_DividendsPreferredStock | 48.7 us-gaap_DividendsPreferredStock | 0 us-gaap_DividendsPreferredStock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) | 159,546,224 us-gaap_CommonStockSharesIssued |  |  |  | 159,546,224 us-gaap_CommonStockSharesIssued | 159,546,224 us-gaap_CommonStockSharesIssued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from issuance of common shares |  |  |  |  | 0 us-gaap_ProceedsFromIssuanceOfCommonStock | 285.3 us-gaap_ProceedsFromIssuanceOfCommonStock | 0 us-gaap_ProceedsFromIssuanceOfCommonStock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Over Allotment Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) |  |  |  |  |  |  |  | 1,350,000 us-gaap_CommonStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_OverAllotmentOptionMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Class A [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Series A Mandatory Convertible Preferred Stock, Class A, Percentage |  |  |  |  | 7.00%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Liquidation Preference Per Share | $ 1,000 us-gaap_PreferredStockLiquidationPreference / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  | $ 1,000 us-gaap_PreferredStockLiquidationPreference / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 1,000 us-gaap_PreferredStockLiquidationPreference / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, per share |  |  |  |  |  |  |  |  |  | $ 17.50 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 17.5000 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 17.50 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | $ 13.6111 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |
Dividends, Preferred Stock | $ 12.8 us-gaap_DividendsPreferredStock / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  | $ 51.2 us-gaap_DividendsPreferredStock / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares, public offering (in shares) |  |  |  |  |  |  |  | 731,250 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Closing price per share (in usd per share) |  |  |  |  |  |  |  | $ 25 us-gaap_SharePrice / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, per share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.150 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.150 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.150 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.150 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.150 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.15 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember | $ 0.15 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember
Percentage increase (decrease) in dividends payable |  |  |  |  |  |  |  |  | 76.00%clf_IncreaseDecreaseInDividendsPayablePercentage / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares, public offering (in shares) |  |  |  |  |  |  |  | 10,350,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) |  |  |  |  |  |  |  | 9,000,000 us-gaap_CommonStockSharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Closing price per share (in usd per share) |  |  |  |  |  |  |  | $ 29.00 us-gaap_SharePrice / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible Preferred Stock, Shares Issued upon Conversion | 28.1480 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  | 28.1480 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible Preferred Stock, Shares Issued Upon Conversion per Depositary Share | 0.7037 clf_ConvertiblePreferredStockSharesIssuedUponConversionperDepositaryShare / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  | 0.7037 clf_ConvertiblePreferredStockSharesIssuedUponConversionperDepositaryShare / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Common Shares Mandatory Convertible Stock Converts to on February 1, 2016 | 20,600,000 clf_ConvertiblePreferredStockTotalSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  | 20,600,000 clf_ConvertiblePreferredStockTotalSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible Preferred Stock, Shares Issued upon Conversion | 34.4840 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  | 34.4840 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Common Shares Mandatory Convertible Stock Converts to on February 1, 2016 | 25,200,000 clf_ConvertiblePreferredStockTotalSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  | 25,200,000 clf_ConvertiblePreferredStockTotalSharesIssuedUponConversion / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_StatementClassOfStockAxis = us-gaap_CommonStockMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock from when Mandatory Convertible Preferred Stock Converts on February 1, 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Par or Stated Value Per Share | $ 0.125 us-gaap_CommonStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = clf_CommonStockFromWhenMandatoryConvertiblePreferredStockConvertsOnFebruary12016Member |  |  |  | $ 0.125 us-gaap_CommonStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = clf_CommonStockFromWhenMandatoryConvertiblePreferredStockConvertsOnFebruary12016Member |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Depositary Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Depositary Share Offering included in Depositary Share Issuance |  |  |  |  |  |  |  | 27,000,000 clf_DepositaryShareOfferingIncludedInDepositaryShareIssuance / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Shares included in Depositary Share Issuance due to Exercise of |  |  |  |  |  |  |  | 2,250,000 clf_NumberofSharesincludedinDepositaryShareIssuanceduetoExerciseofUnderwritersOverallotmentOption / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock, Liquidation Preference Per Share | $ 25 us-gaap_PreferredStockLiquidationPreference / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  | $ 25 us-gaap_PreferredStockLiquidationPreference / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends payable, per share |  |  |  |  |  |  |  |  |  | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 17.5000 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 17.5000 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 17.5000 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 0.44 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember | $ 0.34 us-gaap_DividendsPayableAmountPerShare / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  |  |  |  |
Issuance of common shares, public offering (in shares) |  |  |  |  |  |  |  | 29,250,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = clf_DepositarySharesMember |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Depositary Share Interest in a Share of 7% Series A Mandatory Convertible Preferred Stock, Class A + References No definition available. + Details
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- Definition Face amount or stated value per share of common stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph d -Article 4 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(d)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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