SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Mar. 31, 2019 |
Dec. 31, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Cash and cash equivalents |
$ 430.2
|
$ 823.2
|
$ 757.2
|
$ 978.3
|
Accounts receivable, net |
20.1
|
226.7
|
Ìý
|
Ìý
|
Inventories |
312.7
|
87.9
|
Ìý
|
Ìý
|
Supplies and other inventories |
97.3
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
107.4
|
91.5
|
Ìý
|
Ìý
|
Income tax receivable, current |
117.3
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
41.0
|
39.8
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,126.0
|
1,479.6
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,410.3
|
1,286.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
68.3
|
83.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
121.3
|
121.3
|
Ìý
|
Ìý
|
Deferred income tax assets |
466.6
|
464.8
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
113.8
|
94.9
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
770.0
|
764.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
3,306.3
|
3,529.6
|
Ìý
|
Ìý
|
Accounts payable |
171.7
|
186.8
|
Ìý
|
Ìý
|
Accrued employment costs |
42.1
|
74.0
|
Ìý
|
Ìý
|
Accrued interest |
23.0
|
38.4
|
Ìý
|
Ìý
|
Partnership distribution payable |
43.8
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
113.4
|
125.5
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
394.0
|
468.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
244.2
|
248.7
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
174.4
|
172.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,087.0
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
145.0
|
123.6
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
3,044.6
|
3,105.4
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
261.7
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
3,306.3
|
3,529.6
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
0.0
|
0.0
|
0.0
|
Accounts receivable, net |
(4.0)
|
(4.1)
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
(4.0)
|
(4.1)
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
0.0
|
0.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Deferred income tax assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
(1,478.6)
|
(1,313.1)
|
Ìý
|
Ìý
|
Long-term intercompany notes |
(121.3)
|
(121.3)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
(1,599.9)
|
(1,434.4)
|
Ìý
|
Ìý
|
TOTAL ASSETS |
(1,603.9)
|
(1,438.5)
|
Ìý
|
Ìý
|
Accounts payable |
(4.0)
|
(4.1)
|
Ìý
|
Ìý
|
Accrued employment costs |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
(4.0)
|
(4.1)
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
0.0
|
0.0
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(121.3)
|
(121.3)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(125.3)
|
(125.4)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
(1,478.6)
|
(1,313.1)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
(1,603.9)
|
(1,438.5)
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.7
|
2.7
|
2.6
|
27.3
|
Accounts receivable, net |
0.3
|
0.3
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
11.3
|
12.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
14.3
|
15.9
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
50.8
|
50.8
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
14.7
|
14.6
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Deferred income tax assets |
1.2
|
1.2
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
121.3
|
121.3
|
Ìý
|
Ìý
|
Other non-current assets |
1.4
|
1.5
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
138.6
|
138.6
|
Ìý
|
Ìý
|
TOTAL ASSETS |
203.7
|
205.3
|
Ìý
|
Ìý
|
Accounts payable |
4.1
|
4.2
|
Ìý
|
Ìý
|
Accrued employment costs |
0.1
|
0.1
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
7.1
|
8.2
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
11.3
|
12.5
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
(234.7)
|
(230.0)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
19.8
|
19.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
7.9
|
8.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(195.7)
|
(188.8)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
399.4
|
394.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
203.7
|
205.3
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.7
|
0.7
|
1.0
|
2.1
|
Accounts receivable, net |
16.2
|
221.3
|
Ìý
|
Ìý
|
Inventories |
312.7
|
87.9
|
Ìý
|
Ìý
|
Supplies and other inventories |
97.3
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
106.7
|
91.4
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
21.1
|
16.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
554.7
|
511.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,347.5
|
1,221.9
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
53.6
|
68.4
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
4.1
|
4.1
|
Ìý
|
Ìý
|
Deferred income tax assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
30.1
|
50.8
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
95.8
|
85.4
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
183.6
|
208.7
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,085.8
|
1,942.0
|
Ìý
|
Ìý
|
Accounts payable |
166.4
|
181.4
|
Ìý
|
Ìý
|
Accrued employment costs |
32.6
|
45.4
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
43.8
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
78.4
|
86.7
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
321.2
|
357.0
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
414.8
|
414.4
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
154.6
|
152.1
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
116.0
|
99.5
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
1,006.6
|
1,023.0
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
1,079.2
|
919.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
2,085.8
|
1,942.0
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
426.8
|
819.8
|
$ 753.6
|
$ 948.9
|
Accounts receivable, net |
7.6
|
9.2
|
Ìý
|
Ìý
|
Inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.7
|
0.1
|
Ìý
|
Ìý
|
Income tax receivable, current |
117.3
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
8.6
|
10.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
561.0
|
956.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
12.0
|
13.3
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
117.2
|
117.2
|
Ìý
|
Ìý
|
Deferred income tax assets |
465.4
|
463.6
|
Ìý
|
Ìý
|
Investment in subsidiaries |
1,448.5
|
1,262.3
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
16.6
|
8.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
2,047.7
|
1,851.1
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,620.7
|
2,820.8
|
Ìý
|
Ìý
|
Accounts payable |
5.2
|
5.3
|
Ìý
|
Ìý
|
Accrued employment costs |
9.4
|
28.5
|
Ìý
|
Ìý
|
Accrued interest |
23.0
|
38.4
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
27.9
|
30.6
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
65.5
|
102.8
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
64.1
|
64.3
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,087.0
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
121.3
|
121.3
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
21.1
|
15.3
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
2,359.0
|
2,396.6
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
261.7
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
$ 2,620.7
|
$ 2,820.8
|
Ìý
|
Ìý
|