SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Cash Flows) (Details) - USD ($)
$ in Millions |
9 Months Ended |
|
|
Sep. 30, 2019 |
Sep. 30, 2018 |
Dec. 31, 2018 |
Dec. 31, 2017 |
| Net cash provided (used) by operating activities |
$ 388.1
|
$ 188.7
|
Ìý
|
Ìý
|
| Purchase of property, plant and equipment |
(447.9)
|
(111.4)
|
Ìý
|
Ìý
|
| Deposits for property, plant and equipment |
(12.8)
|
(83.3)
|
Ìý
|
Ìý
|
| Intercompany investing |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Other investing activities |
11.2
|
21.0
|
Ìý
|
Ìý
|
| Net cash provided (used) by investing activities |
(449.5)
|
(173.7)
|
Ìý
|
Ìý
|
| Repurchase of common shares |
(252.9)
|
0.0
|
Ìý
|
Ìý
|
| Dividends paid |
(45.1)
|
0.0
|
Ìý
|
Ìý
|
| Proceeds from issuance of debt |
720.9
|
0.0
|
Ìý
|
Ìý
|
| Debt issuance costs |
(6.8)
|
(1.5)
|
Ìý
|
Ìý
|
| Repurchase of debt |
(729.3)
|
(16.3)
|
Ìý
|
Ìý
|
| Distributions of partnership equity |
(44.2)
|
(44.2)
|
Ìý
|
Ìý
|
| Intercompany financing |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Other financing activities |
(9.5)
|
(45.7)
|
Ìý
|
Ìý
|
| Net cash provided (used) by financing activities |
(366.9)
|
(107.7)
|
Ìý
|
Ìý
|
| EFFECT OF EXCHANGE RATE CHANGES ON CASH |
0.0
|
(2.3)
|
Ìý
|
Ìý
|
| DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN OTHER CURRENT ASSETS RELATED TO DISCONTINUED OPERATIONS |
(428.3)
|
(95.0)
|
Ìý
|
Ìý
|
| LESS: DECREASE IN CASH AND CASH EQUIVALENTS FROM DISCONTINUED OPERATIONS, CLASSIFIED WITHIN OTHER CURRENT ASSETS |
(4.4)
|
(13.8)
|
Ìý
|
Ìý
|
| NET DECREASE IN CASH AND CASH EQUIVALENTS |
(423.9)
|
(81.2)
|
Ìý
|
Ìý
|
| Cash and cash equivalents |
399.3
|
897.1
|
$ 823.2
|
$ 978.3
|
| Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
| Net cash provided (used) by operating activities |
(11.0)
|
(100.6)
|
Ìý
|
Ìý
|
| Purchase of property, plant and equipment |
(1.2)
|
(1.1)
|
Ìý
|
Ìý
|
| Deposits for property, plant and equipment |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Intercompany investing |
(83.6)
|
185.7
|
Ìý
|
Ìý
|
| Other investing activities |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Net cash provided (used) by investing activities |
(84.8)
|
184.6
|
Ìý
|
Ìý
|
| Repurchase of common shares |
(252.9)
|
Ìý
|
Ìý
|
Ìý
|
| Dividends paid |
(45.1)
|
Ìý
|
Ìý
|
Ìý
|
| Proceeds from issuance of debt |
720.9
|
Ìý
|
Ìý
|
Ìý
|
| Debt issuance costs |
(6.8)
|
(1.5)
|
Ìý
|
Ìý
|
| Repurchase of debt |
(729.3)
|
(16.3)
|
Ìý
|
Ìý
|
| Distributions of partnership equity |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Intercompany financing |
0.0
|
(120.7)
|
Ìý
|
Ìý
|
| Other financing activities |
(14.6)
|
(1.9)
|
Ìý
|
Ìý
|
| Net cash provided (used) by financing activities |
(327.8)
|
(140.4)
|
Ìý
|
Ìý
|
| EFFECT OF EXCHANGE RATE CHANGES ON CASH |
Ìý
|
0.0
|
Ìý
|
Ìý
|
| DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN OTHER CURRENT ASSETS RELATED TO DISCONTINUED OPERATIONS |
(423.6)
|
(56.4)
|
Ìý
|
Ìý
|
| LESS: DECREASE IN CASH AND CASH EQUIVALENTS FROM DISCONTINUED OPERATIONS, CLASSIFIED WITHIN OTHER CURRENT ASSETS |
0.0
|
0.0
|
Ìý
|
Ìý
|
| NET DECREASE IN CASH AND CASH EQUIVALENTS |
(423.6)
|
(56.4)
|
Ìý
|
Ìý
|
| Cash and cash equivalents |
396.2
|
892.5
|
819.8
|
948.9
|
| Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
| Net cash provided (used) by operating activities |
402.8
|
425.6
|
Ìý
|
Ìý
|
| Purchase of property, plant and equipment |
(446.7)
|
(110.2)
|
Ìý
|
Ìý
|
| Deposits for property, plant and equipment |
(12.3)
|
(78.1)
|
Ìý
|
Ìý
|
| Intercompany investing |
(1.2)
|
(6.3)
|
Ìý
|
Ìý
|
| Other investing activities |
10.4
|
3.1
|
Ìý
|
Ìý
|
| Net cash provided (used) by investing activities |
(449.8)
|
(191.5)
|
Ìý
|
Ìý
|
| Repurchase of common shares |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Dividends paid |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Proceeds from issuance of debt |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Debt issuance costs |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Repurchase of debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Distributions of partnership equity |
(44.2)
|
(44.2)
|
Ìý
|
Ìý
|
| Intercompany financing |
83.1
|
(188.6)
|
Ìý
|
Ìý
|
| Other financing activities |
7.8
|
(1.5)
|
Ìý
|
Ìý
|
| Net cash provided (used) by financing activities |
46.7
|
(234.3)
|
Ìý
|
Ìý
|
| EFFECT OF EXCHANGE RATE CHANGES ON CASH |
Ìý
|
0.0
|
Ìý
|
Ìý
|
| DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN OTHER CURRENT ASSETS RELATED TO DISCONTINUED OPERATIONS |
(0.3)
|
(0.2)
|
Ìý
|
Ìý
|
| LESS: DECREASE IN CASH AND CASH EQUIVALENTS FROM DISCONTINUED OPERATIONS, CLASSIFIED WITHIN OTHER CURRENT ASSETS |
0.0
|
0.0
|
Ìý
|
Ìý
|
| NET DECREASE IN CASH AND CASH EQUIVALENTS |
(0.3)
|
(0.2)
|
Ìý
|
Ìý
|
| Cash and cash equivalents |
0.4
|
1.9
|
0.7
|
2.1
|
| Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
| Net cash provided (used) by operating activities |
(3.7)
|
(136.3)
|
Ìý
|
Ìý
|
| Purchase of property, plant and equipment |
0.0
|
(0.1)
|
Ìý
|
Ìý
|
| Deposits for property, plant and equipment |
(0.5)
|
(5.2)
|
Ìý
|
Ìý
|
| Intercompany investing |
0.0
|
120.7
|
Ìý
|
Ìý
|
| Other investing activities |
0.8
|
17.9
|
Ìý
|
Ìý
|
| Net cash provided (used) by investing activities |
0.3
|
133.3
|
Ìý
|
Ìý
|
| Repurchase of common shares |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Dividends paid |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Proceeds from issuance of debt |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Debt issuance costs |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Repurchase of debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Distributions of partnership equity |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Intercompany financing |
1.7
|
9.2
|
Ìý
|
Ìý
|
| Other financing activities |
(2.7)
|
(42.3)
|
Ìý
|
Ìý
|
| Net cash provided (used) by financing activities |
(1.0)
|
(33.1)
|
Ìý
|
Ìý
|
| EFFECT OF EXCHANGE RATE CHANGES ON CASH |
Ìý
|
(2.3)
|
Ìý
|
Ìý
|
| DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN OTHER CURRENT ASSETS RELATED TO DISCONTINUED OPERATIONS |
(4.4)
|
(38.4)
|
Ìý
|
Ìý
|
| LESS: DECREASE IN CASH AND CASH EQUIVALENTS FROM DISCONTINUED OPERATIONS, CLASSIFIED WITHIN OTHER CURRENT ASSETS |
(4.4)
|
(13.8)
|
Ìý
|
Ìý
|
| NET DECREASE IN CASH AND CASH EQUIVALENTS |
0.0
|
(24.6)
|
Ìý
|
Ìý
|
| Cash and cash equivalents |
2.7
|
2.7
|
2.7
|
27.3
|
| Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
| Net cash provided (used) by operating activities |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Purchase of property, plant and equipment |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Deposits for property, plant and equipment |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Intercompany investing |
84.8
|
(300.1)
|
Ìý
|
Ìý
|
| Other investing activities |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Net cash provided (used) by investing activities |
84.8
|
(300.1)
|
Ìý
|
Ìý
|
| Repurchase of common shares |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Dividends paid |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Proceeds from issuance of debt |
0.0
|
Ìý
|
Ìý
|
Ìý
|
| Debt issuance costs |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Repurchase of debt |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Distributions of partnership equity |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Intercompany financing |
(84.8)
|
300.1
|
Ìý
|
Ìý
|
| Other financing activities |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Net cash provided (used) by financing activities |
(84.8)
|
300.1
|
Ìý
|
Ìý
|
| EFFECT OF EXCHANGE RATE CHANGES ON CASH |
Ìý
|
0.0
|
Ìý
|
Ìý
|
| DECREASE IN CASH AND CASH EQUIVALENTS, INCLUDING CASH CLASSIFIED WITHIN OTHER CURRENT ASSETS RELATED TO DISCONTINUED OPERATIONS |
0.0
|
0.0
|
Ìý
|
Ìý
|
| LESS: DECREASE IN CASH AND CASH EQUIVALENTS FROM DISCONTINUED OPERATIONS, CLASSIFIED WITHIN OTHER CURRENT ASSETS |
0.0
|
0.0
|
Ìý
|
Ìý
|
| NET DECREASE IN CASH AND CASH EQUIVALENTS |
0.0
|
0.0
|
Ìý
|
Ìý
|
| Cash and cash equivalents |
$ 0.0
|
$ 0.0
|
$ 0.0
|
$ 0.0
|