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
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Net cash provided (used) by operating activities $ (49.3) $ 67.8      
Purchase of property, plant and equipment (42.1) (44.3)      
Deposits for property, plant and equipment (72.3) (5.1)      
Intercompany investing 0.0 0.0      
Proceeds on sales of assets 14.6 1.1      
Net cash used by investing activities (99.8) (48.3)      
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)      
For the Six Months Ended June 30, 2018 (15.3) (1,154.0)      
Distributions of partnership equity 0.0 (8.7)      
Intercompany financing 0.0 0.0      
Other financing activities (8.9) (13.9)      
Net cash used by financing activities (25.7) (23.8)      
EFFECT OF EXCHANGE RATE CHANGES ON CASH (1.0) 2.4      
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (175.8) (1.9)      
LESS: INCREASE IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 40.5      
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (175.8) (42.4)      
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 802.5   $ 978.3 $ 270.4 $ 312.8
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member]          
Net cash provided (used) by operating activities (82.1) (128.8)      
Purchase of property, plant and equipment (0.3) (0.8)      
Deposits for property, plant and equipment 0.0 0.0      
Intercompany investing (35.2) 43.5      
Proceeds on sales of assets 0.0 0.0      
Net cash used by investing activities (35.5) 42.7      
Net proceeds from issuance of common shares   661.3      
Proceeds from issuance of debt   500.0      
Debt issuance costs (1.5) (8.5)      
For the Six Months Ended June 30, 2018 (15.3) (1,154.0)      
Distributions of partnership equity   0.0      
Intercompany financing (14.6) 45.0      
Other financing activities (1.5) (0.6)      
Net cash used by financing activities (32.9) 43.2      
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 (150.5) (42.9)      
LESS: INCREASE 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 (150.5) (42.9)      
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 798.4   948.9 240.5 283.4
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]          
Net cash provided (used) by operating activities 86.8 106.3      
Purchase of property, plant and equipment (24.9) (42.7)      
Deposits for property, plant and equipment (3.6) (5.1)      
Intercompany investing (5.5) (1.1)      
Proceeds on sales of assets 0.4 1.1      
Net cash used by investing activities (33.6) (47.8)      
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 Six Months Ended June 30, 2018 0.0 0.0      
Distributions of partnership equity   (8.7)      
Intercompany financing (52.7) (44.3)      
Other financing activities (1.0) (5.0)      
Net cash used by financing activities (53.7) (58.0)      
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 (0.5) 0.5      
LESS: INCREASE 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 (0.5) 0.5      
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 1.6   2.1 3.0 2.5
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]          
Net cash provided (used) by operating activities (54.0) 90.3      
Purchase of property, plant and equipment (16.9) (0.8)      
Deposits for property, plant and equipment (68.7) 0.0      
Intercompany investing 14.6 (45.0)      
Proceeds on sales of assets 14.2 0.0      
Net cash used by investing activities (56.8) (45.8)      
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 Six Months Ended June 30, 2018 0.0 0.0      
Distributions of partnership equity   0.0      
Intercompany financing 93.4 1.9      
Other financing activities (6.4) (8.3)      
Net cash used by financing activities 87.0 (6.4)      
EFFECT OF EXCHANGE RATE CHANGES ON CASH (1.0) 2.4      
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (24.8) 40.5      
LESS: INCREASE IN CASH AND CASH EQUIVALENTS CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS 0.0 (40.5)      
DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN CURRENT ASSETS OF DISCONTINUED OPERATIONS (24.8) 0.0      
Cash And Cash Equivalents, At Carrying Value, Including Assets Held For Sale 2.5   27.3 26.9 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 26.1 2.6      
Proceeds on sales of assets 0.0 0.0      
Net cash used by investing activities 26.1 2.6      
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 Six Months Ended June 30, 2018 0.0 0.0      
Distributions of partnership equity   0.0      
Intercompany financing (26.1) (2.6)      
Other financing activities 0.0 0.0      
Net cash used by financing activities (26.1) (2.6)      
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 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