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DEBT AND CREDIT FACILITIES (Narrative) (Details) $ / shares in Units, shares in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) shares | Mar. 31, 2017 USD ($) shares | Mar. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares | Dec. 31, 2015 USD ($) | Dec. 19, 2017 USD ($) | Feb. 09, 2017 $ / shares | Sep. 16, 2016 USD ($) | Aug. 10, 2016 $ / shares
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 1,610,700,000 |  |  |  |  |  |
Common shares, par value | $ / shares | $ 0.125 |  |  |  |  | $ 0.125 | $ 0.125 |  |  |  |  |
Common Stock, New Shares, Issued | shares |  | 44,400 | 63,250 |  |  |  |  |  |  |  |  |
Net proceeds from issuance of common shares |  |  |  |  | $ 287,600,000 | $ 661,300,000 | $ 287,400,000 | $ 0 |  |  |  |
Shares Issued, Price Per Share | $ / shares |  |  |  |  |  |  |  |  |  | $ 10.75 |  | $ 6.75
Gain on extinguishment/restructuring of debt |  |  |  | $ (174,300,000) |  | 165,400,000 | (166,300,000) | (392,900,000) |  |  |  |
Debt redemption premiums paid |  |  |  |  |  | 110,000,000 | 17,400,000 |  |  |  |  |
Debt issuance costs |  |  |  |  |  | 28,600,000 | 5,200,000 | $ 33,600,000 |  |  |  |
Adjustments to Additional Paid in Capital, Convertible Debt with Conversion Feature |  |  |  |  |  | $ 83,400,000 |  |  |  |  |  |
Long-term Debt, Current Maturities |  |  |  |  |  |  | 17,500,000 |  |  |  |  |
Common Shares Issued in Debt to Equity Exchange | shares | 8,200 |  |  |  |  | 8,200 |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | $ 512,200,000 |  |  |  |  |
$1.075 Billion 5.75% 2025 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt |  | $ 575,000,000 | $ 500,000,000 |  |  |  |  |  |  |  |  |
Imputed interest rate | 6.01% |  |  |  |  | 6.01% |  |  |  |  |  |
Redemption Price of 35 percent or less of Outstanding |  |  |  |  |  | 1.0575 |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.75% |  |  |  |  | 5.75% |  |  |  |  |  |
Debt issuance, discount rate | 97.00% |  |  |  |  | 97.00% |  |  |  |  |  |
Repurchase price if triggering event occurs |  |  |  |  |  | 1.01 |  |  |  |  |  |
In the Event of Default Amount that will Accelerate |  |  |  |  |  | 0.25 |  |  |  |  |  |
Debt Instrument, Par Value | $ 1,075,000,000 |  |  |  |  | $ 1,075,000,000 |  |  |  |  |  |
Note Redemption Price, Percent of Principal Amount to be Redeemed | 100.00% |  |  |  |  | 100.00% |  |  |  |  |  |
Amount in aggregate that can be redeemed on or prior to March 1, 2020 |  |  |  |  |  | 0.3500 |  |  |  |  |  |
Debt issuance costs |  |  |  |  |  | $ 12,400,000 |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 11,300,000 |  |  |  |  | 11,300,000 |  |  |  |  |  |
Debt Instrument, Unamortized Discount | 16,500,000 |  |  |  |  | $ 16,500,000 |  |  |  |  |  |
$400 Million 4.875% 2024 Senior Notes [Domain] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt | $ 400,000,000 |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 5.00% |  |  |  |  | 5.00% |  |  |  |  |  |
Redemption Price of 35 percent or less of Outstanding |  |  |  |  |  | 1.0488 |  |  |  |  |  |
Amount to Remain Outstanding Prior to March 31, 2018 |  |  |  |  |  | 0.6500 |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.875% |  |  |  |  | 4.875% |  |  |  |  |  |
Debt issuance, discount rate | 99.00% |  |  |  |  | 99.00% |  |  |  |  |  |
Repurchase price if triggering event occurs |  |  |  |  |  | 1.01 |  |  |  |  |  |
Initial Redemption Price |  |  |  |  |  | 1.02438 |  |  |  |  |  |
In the Event of Default Amount that will Accelerate |  |  |  |  |  | 0.25 |  |  |  |  |  |
Debt Instrument, Par Value | $ 400,000,000 |  |  |  |  | $ 400,000,000 |  |  |  |  |  |
Note Redemption Price, Percent of Principal Amount to be Redeemed | 100.00% |  |  |  |  | 100.00% |  |  |  |  |  |
Redemption amount prior to January 15, 2021 |  |  |  |  |  | 10.00% |  |  |  |  |  |
Secondary Redemption Price |  |  |  |  |  | 1.03 |  |  |  |  |  |
Amount in aggregate that can be redeemed on or prior to March 31, 2018 |  |  |  |  |  | 0.3500 |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 7,100,000 |  |  |  |  | $ 7,100,000 |  |  |  |  |  |
Debt Instrument, Unamortized Discount | 2,600,000 |  |  |  |  | $ 2,600,000 |  |  |  |  |  |
$316 Million 1.5% 2025 Senior Notes [Domain] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Debt | $ 316,300,000 |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 6.26% |  |  |  |  | 6.26% |  |  |  |  |  |
Convertible debt, initial conversion price | $ 8.17 |  |  |  |  | $ 8.17 |  |  |  |  |  |
Convertible debt, common shares exchanged | $ 122.4365 |  |  |  |  | $ 122.4365 |  |  |  |  |  |
Common shares, par value | $ / shares | $ 0.125 |  |  |  |  | $ 0.125 |  |  |  |  |  |
Convertible debt, principal increment | $ 1,000 |  |  |  |  | $ 1,000 |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 1.50% |  |  |  |  | 1.50% |  |  |  |  |  |
Debt Instrument, Par Value | $ 316,300,000 |  |  |  |  | $ 316,300,000 |  |  |  |  |  |
Note Redemption Price, Percent of Principal Amount to be Redeemed | 100.00% |  |  |  |  | 100.00% |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 6,600,000 |  |  |  |  | $ 6,600,000 |  |  | $ 9,100,000 |  |  |
Debt Instrument, Unamortized Discount | 85,600,000 |  |  |  |  | 85,600,000 |  |  |  |  |  |
Adjustments to Additional Paid in Capital, Convertible Debt with Conversion Feature |  |  |  |  |  | 83,400,000 |  |  |  |  |  |
Debt issuance costs to APIC |  |  |  |  |  | 2,500,000 |  |  |  |  |  |
$316 Million 1.5% 2025 Senior Notes [Domain] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Convertible debt, conversion price percentage | 0.98 |  |  |  |  | 0.98 |  |  |  |  |  |
$316 Million 1.5% 2025 Senior Notes [Domain] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Convertible debt, conversion price percentage | $ 1.30 |  |  |  |  | 1.30 |  |  |  |  |  |
$540 Million 8.25% 2020 First Lien Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate |  |  |  |  |  |  | 9.97% |  |  |  |  |
Extinguishment of Debt, Amount |  |  | $ 35,600,000 |  |  | 540,000,000 |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | $ 540,000,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  | $ 93,500,000 |  |  |  |  |  |
Unamortized Debt Issuance Expense |  |  |  |  |  |  | 8,000,000 |  |  |  |  |
Debt Instrument, Unamortized Discount |  |  |  |  |  |  | 25,700,000 |  |  |  |  |
$218.5 Million 8.00% 2020 1.5 Lien Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 8.00% |  |  |  |  | 8.00% |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 218,500,000 |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 218,500,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  | (45,100,000) |  |  |  |  |  |
Unamortized Debt Issuance Expense |  |  |  |  |  |  | 0 |  |  |  |  |
Debt Instrument, Unamortized Discount |  |  |  |  |  |  | $ 65,700,000 |  |  |  |  |
$544.2 Million 7.75% 2020 Second Lien Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate |  |  |  |  |  |  | 15.55% |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | 430,100,000 |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | $ 430,100,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  | 104,500,000 | (6,900,000) |  |  |  |  |
Unamortized Debt Issuance Expense |  |  |  |  |  |  | 5,800,000 |  |  |  |  |
Debt Instrument, Unamortized Discount |  |  |  |  |  |  | 85,200,000 |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | 114,100,000 |  |  |  |  |
Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 648,600,000 |  |  |  |  |  |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.95% |  |  |  |  | 3.95% |  |  |  |  |  |
Gain on extinguishment/restructuring of debt | $ (19,900,000) |  |  |  |  |  | (1,800,000) |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | $ 17,600,000 |  |  |  |  |
$400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 5.98% |  |  |  |  | 5.98% | 5.98% |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 136,700,000 |  |  |  |  |  |
Debt Instrument, Par Value | $ 88,900,000 |  |  |  |  | 88,900,000 | $ 225,600,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  | $ 7,800,000 | (28,300,000) |  |  |  |  |
Basis points | 35 |  |  |  |  | 35 |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 200,000 |  |  |  |  | $ 200,000 | 600,000 |  |  |  |  |
Debt Instrument, Unamortized Discount | $ 100,000 |  |  |  |  | $ 100,000 | 500,000 |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | $ 65,100,000 |  |  |  |  |
$500 Million 4.80% 2020 Senior Notes[Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 4.83% |  |  |  |  | 4.83% | 4.83% |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 114,400,000 |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.80% |  |  |  |  | 4.80% |  |  |  |  |  |
Debt Instrument, Par Value | $ 122,400,000 |  |  |  |  | $ 122,400,000 | $ 236,800,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt | (600,000) |  |  |  |  | 1,900,000 | (19,500,000) |  |  |  |  |
Debt Redemption Expenses | 2,500,000 |  |  |  |  |  |  |  |  |  |  |
Unamortized Debt Issuance Expense | 300,000 |  |  |  |  | 300,000 | 700,000 |  |  |  |  |
Debt Instrument, Unamortized Discount | $ 100,000 |  |  |  |  | $ 100,000 | 200,000 |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | $ 44,700,000 |  |  |  |  |
$700 Million 4.875% 2021 Senior Notes[Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 4.89% |  |  |  |  | 4.89% | 4.89% |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 171,000,000 |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 4.875% |  |  |  |  | 4.875% |  |  |  |  |  |
Debt Instrument, Par Value | $ 138,400,000 |  |  |  |  | $ 138,400,000 | $ 309,400,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  | $ 2,800,000 | (33,300,000) |  |  |  |  |
Basis points | 25 |  |  |  |  | 25 |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 300,000 |  |  |  |  | $ 300,000 | 1,000,000 |  |  |  |  |
Debt Instrument, Unamortized Discount | $ 100,000 |  |  |  |  | $ 100,000 | 200,000 |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | $ 76,300,000 |  |  |  |  |
$800 Million 6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Imputed interest rate | 6.34% |  |  |  |  | 6.34% | 6.34% |  |  |  |  |
Debt Instrument, Par Value | $ 298,400,000 |  |  |  |  | $ 298,400,000 | $ 298,400,000 |  |  |  |  |
Gain on extinguishment/restructuring of debt |  |  |  |  |  |  | (84,500,000) |  |  |  |  |
Basis points | 40 |  |  |  |  | 40 |  |  |  |  |  |
Unamortized Debt Issuance Expense | $ 2,400,000 |  |  |  |  | $ 2,400,000 | 2,500,000 |  |  |  |  |
Debt Instrument, Unamortized Discount | $ 3,400,000 |  |  |  |  | 3,400,000 | 3,400,000 |  |  |  |  |
Debt Instrument, Face Amount Exchanged |  |  |  |  |  |  | 194,400,000 |  |  |  |  |
Unsecured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  | $ 422,200,000 |  |  |  |  |  |
Repurchase price if triggering event occurs |  |  |  |  |  | 1.01 |  |  |  |  |  |
Note Redemption Price, Percent of Principal Amount to be Redeemed | 100.00% |  |  |  |  | 100.00% |  |  |  |  |  |
Unsecured Debt [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Time Period During Which Senior Notes May Be Redeemed at Company's Option, Number of Days From When Prior Notice Sent to Holders |  |  |  |  |  | 60 days |  |  |  |  |  |
Unsecured Debt [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Time Period During Which Senior Notes May Be Redeemed at Company's Option, Number of Days From When Prior Notice Sent to Holders |  |  |  |  |  | 30 days |  |  |  |  |  |
ABL Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value | $ 550,000,000 |  |  |  |  | $ 550,000,000 |  |  |  |  |  |
U.S. Tranche | 450,000,000 |  |  |  |  | 450,000,000 |  |  |  |  |  |
Sublimit for Issuers of Letters of Credit for U.S. Tranche | 250,000,000 |  |  |  |  | 250,000,000 |  |  |  |  |  |
Sublimit for U.S. Swingline Loans | 100,000,000 |  |  |  |  | 100,000,000 |  |  |  |  |  |
Australian Tranche | 100,000,000 |  |  |  |  | 100,000,000 |  |  |  |  |  |
Sublimit for Issuance of Letters of Credit for Australian Tranche | 50,000,000 |  |  |  |  | 50,000,000 |  |  |  |  |  |
Sublimit for Australian Swingline Loans | 20,000,000 |  |  |  |  | $ 20,000,000 |  |  |  |  |  |
LIBOR Rate Based on a One-month interest period plus 1 percent |  |  |  |  |  | 0.01 |  |  |  |  |  |
Fixed Charge Coverage Ratio |  |  |  |  |  | 1.0 |  |  |  |  |  |
Credit facility, amount outstanding | 0 |  |  |  |  | $ 0 | 0 |  |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity | 226,700,000 |  |  |  |  | 226,700,000 | 227,000,000 |  |  |  |  |
Revolving credit facility, borrowing capacity | 273,200,000 |  |  |  |  | $ 273,200,000 | 333,000,000 |  |  |  |  |
ABL Facility [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Base Rate |  |  |  |  |  | 0.01 |  |  |  |  |  |
Letter of Credit [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Credit facility, amount outstanding | 46,500,000 |  |  |  |  | $ 46,500,000 | $ 106,000,000 |  |  |  |  |
Debt Repurchase [Member] | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Redemption Price |  |  |  |  |  |  |  |  |  |  | $ 301,000,000 |
Debt Instrument, Par Value |  |  |  |  | 283,600,000 |  |  |  |  |  |  |
Debt Repurchase [Member] | $500 Million 4.80% 2020 Senior Notes[Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  | $ 5,000,000 |  |  |  |  |  |  |
Exchange of Debt for Equity [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gain on extinguishment/restructuring of debt | (11,300,000) |  |  |  |  |  |  |  |  |  |  |
Exchange of Debt for Equity [Member] | $218.5 Million 8.00% 2020 1.5 Lien Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 20,100,000 |  |  |  |  | 20,100,000 |  |  |  |  |  |
Exchange of Debt for Equity [Member] | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 10,000,000 |  |  |  |  | 10,000,000 |  |  |  |  |  |
Exchange of Debt for Equity [Member] | $700 Million 4.875% 2021 Senior Notes[Member] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount Exchanged | 26,800,000 |  |  |  |  | 26,800,000 |  |  |  |  |  |
Equity [Member] | $316 Million 1.5% 2025 Senior Notes [Domain] |  |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Unamortized Discount | $ 85,900,000 |  |  |  |  | $ 85,900,000 |  |  |  |  |  |

X
- Definition Amount in aggregate that can be redeemed on or prior to March 1, 2020 + References No definition available. + Details
Name: | clf_AmountinaggregatethatcanberedeemedonorpriortoMarch12020
Namespace Prefix: | clf_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount in aggregate that can be redeemed on or prior to March 31, 2018 + References No definition available. + Details
Name: | clf_AmountinaggregatethatcanberedeemedonorpriortoMarch312018
Namespace Prefix: | clf_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount to Remain Outstanding Prior to March 31, 2018 + References No definition available. + Details
Name: | clf_AmounttoRemainOutstandingPriortoMarch312018
Namespace Prefix: | clf_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Australian Tranche + References No definition available. + Details
Name: | clf_AustralianTranche
Namespace Prefix: | clf_
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