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