Quarterly report pursuant to Section 13 or 15(d)

SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Cash Flows) (Details)

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SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Cash Flows) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Net Cash Provided by (Used in) Operating Activities $ (142.9) $ (25.1)    
Property, Plant and Equipment, Cash Additions (12.4) (25.9)    
Purchase of property, plant and equipment (59.0) (2.0)    
Intercompany investments 0.0 0.0    
Other investing activities 0.0 0.5    
Net Cash Provided by (Used in) Investing Activities (71.4) (27.4)    
Net proceeds from issuance of common shares 0.0 661.3    
Proceeds from issuance of debt 0.0 500.0    
Debt issuance costs (1.5) (8.5)    
Gain (Loss) on Repurchase of Debt Instrument   (1,115.5)    
Distributions of partnership equity 0.0 (8.7)    
Intercompany financing 0.0 0.0    
Other financing activities (5.5) (5.6)    
Net Cash Provided by (Used in) Financing Activities (7.0) 23.0    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.2 1.4    
Cash and Cash Equivalents, Period Increase (Decrease) (221.1) (28.1)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 786.6 295.3 $ 1,007.7 $ 323.4
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member]        
Net Cash Provided by (Used in) Operating Activities (54.7) (99.5)    
Property, Plant and Equipment, Cash Additions 0.0 (0.8)    
Purchase of property, plant and equipment 0.0 0.0    
Intercompany investments (137.7) (56.5)    
Other investing activities   0.0    
Net Cash Provided by (Used in) Investing Activities (137.7) (57.3)    
Net proceeds from issuance of common shares   661.3    
Proceeds from issuance of debt   500.0    
Debt issuance costs (1.5) (8.5)    
Gain (Loss) on Repurchase of Debt Instrument   (1,115.5)    
Distributions of partnership equity   0.0    
Intercompany financing 0.0 45.1    
Other financing activities (1.4) (0.5)    
Net Cash Provided by (Used in) Financing Activities (2.9) 81.9    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
Cash and Cash Equivalents, Period Increase (Decrease) (195.3) (74.9)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 753.6 208.5 948.9 283.4
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]        
Net Cash Provided by (Used in) Operating Activities (62.8) (19.7)    
Property, Plant and Equipment, Cash Additions (8.1) (24.9)    
Purchase of property, plant and equipment (0.8) (2.0)    
Intercompany investments (4.8) (0.5)    
Other investing activities   0.5    
Net Cash Provided by (Used in) Investing Activities (13.7) (26.9)    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
Gain (Loss) on Repurchase of Debt Instrument   0.0    
Distributions of partnership equity   (8.7)    
Intercompany financing 75.9 55.8    
Other financing activities (0.5) (0.7)    
Net Cash Provided by (Used in) Financing Activities 75.4 46.4    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
Cash and Cash Equivalents, Period Increase (Decrease) (1.1) (0.2)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 1.0 2.3 2.1 2.5
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]        
Net Cash Provided by (Used in) Operating Activities (25.4) 94.1    
Property, Plant and Equipment, Cash Additions (4.3) (0.2)    
Purchase of property, plant and equipment (58.2) 0.0    
Intercompany investments 0.0 (45.0)    
Other investing activities   0.0    
Net Cash Provided by (Used in) Investing Activities (62.5) (45.2)    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
Gain (Loss) on Repurchase of Debt Instrument   0.0    
Distributions of partnership equity   0.0    
Intercompany financing 66.6 1.1    
Other financing activities (3.6) (4.4)    
Net Cash Provided by (Used in) Financing Activities 63.0 (3.3)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.2 1.4    
Cash and Cash Equivalents, Period Increase (Decrease) (24.7) 47.0    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 32.0 84.5 56.7 37.5
Consolidation, Eliminations [Member]        
Net Cash Provided by (Used in) Operating Activities 0.0 0.0    
Property, Plant and Equipment, Cash Additions 0.0 0.0    
Purchase of property, plant and equipment 0.0 0.0    
Intercompany investments 142.5 102.0    
Other investing activities   0.0    
Net Cash Provided by (Used in) Investing Activities 142.5 102.0    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
Gain (Loss) on Repurchase of Debt Instrument   0.0    
Distributions of partnership equity   0.0    
Intercompany financing (142.5) (102.0)    
Other financing activities 0.0 0.0    
Net Cash Provided by (Used in) Financing Activities (142.5) (102.0)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
Cash and Cash Equivalents, Period Increase (Decrease) 0.0 0.0    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale $ 0.0 $ 0.0 $ 0.0 $ 0.0