SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2018 |
Jun. 30, 2018 |
Dec. 31, 2017 |
Cash and cash equivalents |
$ 377.2
|
Ìý
|
$ 823.2
|
$ 802.5
|
$ 978.3
|
Accounts receivable, net |
193.1
|
Ìý
|
226.7
|
Ìý
|
Ìý
|
Inventories |
219.0
|
Ìý
|
87.9
|
Ìý
|
Ìý
|
Supplies and other inventories |
110.8
|
Ìý
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
118.3
|
Ìý
|
91.5
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.7
|
Ìý
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
42.3
|
Ìý
|
39.8
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,119.4
|
Ìý
|
1,479.6
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,597.3
|
Ìý
|
1,286.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
52.2
|
Ìý
|
83.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
62.7
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
Deferred income tax assets |
443.3
|
Ìý
|
464.8
|
Ìý
|
Ìý
|
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
118.3
|
Ìý
|
94.9
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
676.5
|
Ìý
|
764.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
3,393.2
|
Ìý
|
3,529.6
|
Ìý
|
Ìý
|
Accounts payable |
188.1
|
Ìý
|
186.8
|
Ìý
|
Ìý
|
Accrued employment costs |
58.4
|
Ìý
|
74.0
|
Ìý
|
Ìý
|
Accrued interest |
31.3
|
Ìý
|
38.4
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.1
|
Ìý
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
115.5
|
Ìý
|
125.5
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
437.4
|
Ìý
|
468.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
239.3
|
Ìý
|
248.7
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
176.7
|
Ìý
|
172.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,104.5
|
Ìý
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
149.7
|
Ìý
|
123.6
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
3,107.6
|
Ìý
|
3,105.4
|
Ìý
|
Ìý
|
TOTAL EQUITY |
285.6
|
$ 261.7
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
3,393.2
|
Ìý
|
3,529.6
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
Ìý
|
0.0
|
0.0
|
0.0
|
Accounts receivable, net |
(4.1)
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
Inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
(4.1)
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deferred income tax assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |
(1,736.4)
|
Ìý
|
(1,313.1)
|
Ìý
|
Ìý
|
Long-term intercompany notes |
(121.3)
|
Ìý
|
(121.3)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
(1,857.7)
|
Ìý
|
(1,434.4)
|
Ìý
|
Ìý
|
TOTAL ASSETS |
(1,861.8)
|
Ìý
|
(1,438.5)
|
Ìý
|
Ìý
|
Accounts payable |
(4.1)
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
Accrued employment costs |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
(4.1)
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(121.3)
|
Ìý
|
(121.3)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(125.4)
|
Ìý
|
(125.4)
|
Ìý
|
Ìý
|
TOTAL EQUITY |
(1,736.4)
|
Ìý
|
(1,313.1)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
(1,861.8)
|
Ìý
|
(1,438.5)
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.6
|
Ìý
|
2.7
|
2.5
|
27.3
|
Accounts receivable, net |
0.3
|
Ìý
|
0.3
|
Ìý
|
Ìý
|
Inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
9.7
|
Ìý
|
12.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
12.6
|
Ìý
|
15.9
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
50.8
|
Ìý
|
50.8
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
14.8
|
Ìý
|
14.6
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deferred income tax assets |
1.2
|
Ìý
|
1.2
|
Ìý
|
Ìý
|
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
121.3
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
Other non-current assets |
1.4
|
Ìý
|
1.5
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
138.7
|
Ìý
|
138.6
|
Ìý
|
Ìý
|
TOTAL ASSETS |
202.1
|
Ìý
|
205.3
|
Ìý
|
Ìý
|
Accounts payable |
4.2
|
Ìý
|
4.2
|
Ìý
|
Ìý
|
Accrued employment costs |
0.1
|
Ìý
|
0.1
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
7.2
|
Ìý
|
8.2
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
11.5
|
Ìý
|
12.5
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
(239.5)
|
Ìý
|
(230.0)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
19.6
|
Ìý
|
19.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
7.1
|
Ìý
|
8.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(201.3)
|
Ìý
|
(188.8)
|
Ìý
|
Ìý
|
TOTAL EQUITY |
403.4
|
Ìý
|
394.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
202.1
|
Ìý
|
205.3
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.4
|
Ìý
|
0.7
|
1.6
|
2.1
|
Accounts receivable, net |
188.8
|
Ìý
|
221.3
|
Ìý
|
Ìý
|
Inventories |
219.0
|
Ìý
|
87.9
|
Ìý
|
Ìý
|
Supplies and other inventories |
110.8
|
Ìý
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
118.1
|
Ìý
|
91.4
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
24.1
|
Ìý
|
16.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
661.2
|
Ìý
|
511.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,534.9
|
Ìý
|
1,221.9
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
37.4
|
Ìý
|
68.4
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
4.1
|
Ìý
|
4.1
|
Ìý
|
Ìý
|
Deferred income tax assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |
35.2
|
Ìý
|
50.8
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
100.8
|
Ìý
|
85.4
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
177.5
|
Ìý
|
208.7
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,373.6
|
Ìý
|
1,942.0
|
Ìý
|
Ìý
|
Accounts payable |
180.2
|
Ìý
|
181.4
|
Ìý
|
Ìý
|
Accrued employment costs |
42.2
|
Ìý
|
45.4
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.1
|
Ìý
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
82.1
|
Ìý
|
86.7
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
348.6
|
Ìý
|
357.0
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
414.8
|
Ìý
|
414.4
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
157.1
|
Ìý
|
152.1
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
120.1
|
Ìý
|
99.5
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
1,040.6
|
Ìý
|
1,023.0
|
Ìý
|
Ìý
|
TOTAL EQUITY |
1,333.0
|
Ìý
|
919.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
2,373.6
|
Ìý
|
1,942.0
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
374.2
|
Ìý
|
819.8
|
$ 798.4
|
$ 948.9
|
Accounts receivable, net |
8.1
|
Ìý
|
9.2
|
Ìý
|
Ìý
|
Inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.2
|
Ìý
|
0.1
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.7
|
Ìý
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
8.5
|
Ìý
|
10.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
449.7
|
Ìý
|
956.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
11.6
|
Ìý
|
13.3
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
58.6
|
Ìý
|
117.2
|
Ìý
|
Ìý
|
Deferred income tax assets |
442.1
|
Ìý
|
463.6
|
Ìý
|
Ìý
|
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures |
1,701.2
|
Ìý
|
1,262.3
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
16.1
|
Ìý
|
8.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
2,218.0
|
Ìý
|
1,851.1
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,679.3
|
Ìý
|
2,820.8
|
Ìý
|
Ìý
|
Accounts payable |
7.8
|
Ìý
|
5.3
|
Ìý
|
Ìý
|
Accrued employment costs |
16.1
|
Ìý
|
28.5
|
Ìý
|
Ìý
|
Accrued interest |
31.3
|
Ìý
|
38.4
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
26.2
|
Ìý
|
30.6
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
81.4
|
Ìý
|
102.8
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
64.0
|
Ìý
|
64.3
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,104.5
|
Ìý
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
121.3
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
22.5
|
Ìý
|
15.3
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
2,393.7
|
Ìý
|
2,396.6
|
Ìý
|
Ìý
|
TOTAL EQUITY |
285.6
|
Ìý
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
$ 2,679.3
|
Ìý
|
$ 2,820.8
|
Ìý
|
Ìý
|