Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Cash Flows) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Net cash provided (used) by operating activities $ 188.7 $ 206.7    
Purchase of property, plant and equipment (111.4) (62.7)    
Deposits for property, plant and equipment (83.3) (16.2)    
Intercompany investing 0.0 0.0    
Proceeds from Sale of Other Assets, Investing Activities 18.5 2.2    
Proceeds on sales of assets 2.5 (7.7)    
Net cash used by investing activities (173.7) (84.4)    
Net proceeds from issuance of common shares 0.0 661.3    
Proceeds from issuance of debt 0.0 1,057.8    
Debt issuance costs (1.5) (12.0)    
For the Nine Months Ended September 30, 2018 (16.3) (1,720.7)    
Payments for Repurchase of Redeemable Noncontrolling Interest 0.0 105.0    
Distributions of partnership equity (44.2) (53.0)    
Intercompany financing 0.0 0.0    
Other financing activities (45.7) (17.0)    
Net cash used by financing activities (107.7) (188.6)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH (2.3) 3.7    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (95.0) (62.6)    
LESS: INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (13.8) 23.1    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (81.2) (85.7)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 897.1 227.1 $ 978.3 $ 312.8
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member]        
Net cash provided (used) by operating activities (100.6) (162.9)    
Purchase of property, plant and equipment (1.1) (1.8)    
Deposits for property, plant and equipment 0.0 0.0    
Intercompany investing 185.7 160.6    
Proceeds from Sale of Other Assets, Investing Activities 0.0 0.0    
Proceeds on sales of assets 0.0 (7.7)    
Net cash used by investing activities 184.6 151.1    
Net proceeds from issuance of common shares   661.3    
Proceeds from issuance of debt   1,057.8    
Debt issuance costs (1.5) (12.0)    
For the Nine Months Ended September 30, 2018 (16.3) (1,720.7)    
Payments for Repurchase of Redeemable Noncontrolling Interest   105.0    
Distributions of partnership equity 0.0 0.0    
Intercompany financing (120.7) 45.0    
Other financing activities (1.9) (0.6)    
Net cash used by financing activities (140.4) (74.2)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (56.4) (86.0)    
LESS: INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (56.4) (86.0)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 892.5 197.4 948.9 283.4
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]        
Net cash provided (used) by operating activities 425.6 293.9    
Purchase of property, plant and equipment (110.2) (59.4)    
Deposits for property, plant and equipment (78.1) (11.3)    
Intercompany investing (6.3) (6.5)    
Proceeds from Sale of Other Assets, Investing Activities 0.6 2.2    
Proceeds on sales of assets 2.5 0.0    
Net cash used by investing activities (191.5) (75.0)    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
For the Nine Months Ended September 30, 2018 0.0 0.0    
Payments for Repurchase of Redeemable Noncontrolling Interest   0.0    
Distributions of partnership equity (44.2) (53.0)    
Intercompany financing (188.6) (162.2)    
Other financing activities (1.5) (3.8)    
Net cash used by financing activities (234.3) (219.0)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (0.2) (0.1)    
LESS: INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (0.2) (0.1)    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 1.9 2.4 2.1 2.5
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]        
Net cash provided (used) by operating activities (136.3) 75.7    
Purchase of property, plant and equipment (0.1) (1.5)    
Deposits for property, plant and equipment (5.2) (4.9)    
Intercompany investing 120.7 (45.0)    
Proceeds from Sale of Other Assets, Investing Activities 17.9 0.0    
Proceeds on sales of assets 0.0 0.0    
Net cash used by investing activities 133.3 (51.4)    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
For the Nine Months Ended September 30, 2018 0.0 0.0    
Payments for Repurchase of Redeemable Noncontrolling Interest   0.0    
Distributions of partnership equity 0.0 0.0    
Intercompany financing 9.2 8.1    
Other financing activities (42.3) (12.6)    
Net cash used by financing activities (33.1) (4.5)    
EFFECT OF EXCHANGE RATE CHANGES ON CASH (2.3) 3.7    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (38.4) 23.5    
LESS: INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (13.8) (23.1)    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (24.6) 0.4    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 2.7 27.3 27.3 26.9
Consolidation, Eliminations [Member]        
Net cash provided (used) by operating activities 0.0 0.0    
Purchase of property, plant and equipment 0.0 0.0    
Deposits for property, plant and equipment 0.0 0.0    
Intercompany investing (300.1) (109.1)    
Proceeds from Sale of Other Assets, Investing Activities 0.0 0.0    
Proceeds on sales of assets 0.0 0.0    
Net cash used by investing activities (300.1) (109.1)    
Net proceeds from issuance of common shares   0.0    
Proceeds from issuance of debt   0.0    
Debt issuance costs 0.0 0.0    
For the Nine Months Ended September 30, 2018 0.0 0.0    
Payments for Repurchase of Redeemable Noncontrolling Interest   0.0    
Distributions of partnership equity 0.0 0.0    
Intercompany financing 300.1 109.1    
Other financing activities 0.0 0.0    
Net cash used by financing activities 300.1 109.1    
EFFECT OF EXCHANGE RATE CHANGES ON CASH 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 0.0    
LESS: INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 0.0    
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 0.0    
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale $ 0.0 $ 0.0 $ 0.0 $ 0.0