SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2018 |
Sep. 30, 2018 |
Dec. 31, 2017 |
Cash and cash equivalents |
$ 399.3
|
Ìý
|
Ìý
|
$ 823.2
|
$ 897.1
|
$ 978.3
|
Accounts receivable, net |
164.9
|
Ìý
|
Ìý
|
226.7
|
Ìý
|
Ìý
|
Finished goods inventories |
162.2
|
Ìý
|
Ìý
|
77.8
|
Ìý
|
Ìý
|
Work-in-process inventories |
55.2
|
Ìý
|
Ìý
|
10.1
|
Ìý
|
Ìý
|
Supplies and other inventories |
110.8
|
Ìý
|
Ìý
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
72.8
|
Ìý
|
Ìý
|
91.5
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.7
|
Ìý
|
Ìý
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
40.7
|
Ìý
|
Ìý
|
39.8
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,064.6
|
Ìý
|
Ìý
|
1,479.6
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,769.9
|
Ìý
|
Ìý
|
1,286.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
41.6
|
Ìý
|
Ìý
|
83.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
62.7
|
Ìý
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
Deferred income taxes |
437.5
|
Ìý
|
Ìý
|
464.8
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
114.9
|
Ìý
|
Ìý
|
94.9
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
656.7
|
Ìý
|
Ìý
|
764.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
3,491.2
|
Ìý
|
Ìý
|
3,529.6
|
Ìý
|
Ìý
|
Accounts payable |
212.8
|
Ìý
|
Ìý
|
186.8
|
Ìý
|
Ìý
|
Accrued employment costs |
57.3
|
Ìý
|
Ìý
|
74.0
|
Ìý
|
Ìý
|
Accrued interest |
34.1
|
Ìý
|
Ìý
|
38.4
|
Ìý
|
Ìý
|
Derivative liabilities |
32.6
|
Ìý
|
Ìý
|
3.7
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
Ìý
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
121.7
|
Ìý
|
Ìý
|
121.8
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
458.5
|
Ìý
|
Ìý
|
468.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
233.2
|
Ìý
|
Ìý
|
248.7
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
179.1
|
Ìý
|
Ìý
|
172.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,109.1
|
Ìý
|
Ìý
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
151.4
|
Ìý
|
Ìý
|
123.6
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
3,131.3
|
Ìý
|
Ìý
|
3,105.4
|
Ìý
|
Ìý
|
TOTAL EQUITY |
359.9
|
$ 285.6
|
$ 261.7
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
3,491.2
|
Ìý
|
Ìý
|
3,529.6
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
Ìý
|
Ìý
|
0.0
|
0.0
|
0.0
|
Accounts receivable, net |
(4.1)
|
Ìý
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
Finished goods inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Work-in-process inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
(4.1)
|
Ìý
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deferred income taxes |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
(1,820.2)
|
Ìý
|
Ìý
|
(1,313.1)
|
Ìý
|
Ìý
|
Long-term intercompany notes |
(121.3)
|
Ìý
|
Ìý
|
(121.3)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
(1,941.5)
|
Ìý
|
Ìý
|
(1,434.4)
|
Ìý
|
Ìý
|
TOTAL ASSETS |
(1,945.6)
|
Ìý
|
Ìý
|
(1,438.5)
|
Ìý
|
Ìý
|
Accounts payable |
(4.1)
|
Ìý
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
Accrued employment costs |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative liabilities |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
(4.1)
|
Ìý
|
Ìý
|
(4.1)
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(121.3)
|
Ìý
|
Ìý
|
(121.3)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(125.4)
|
Ìý
|
Ìý
|
(125.4)
|
Ìý
|
Ìý
|
TOTAL EQUITY |
(1,820.2)
|
Ìý
|
Ìý
|
(1,313.1)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
(1,945.6)
|
Ìý
|
Ìý
|
(1,438.5)
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.7
|
Ìý
|
Ìý
|
2.7
|
2.7
|
27.3
|
Accounts receivable, net |
0.2
|
Ìý
|
Ìý
|
0.3
|
Ìý
|
Ìý
|
Finished goods inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Work-in-process inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
8.5
|
Ìý
|
Ìý
|
12.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
11.4
|
Ìý
|
Ìý
|
15.9
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
50.8
|
Ìý
|
Ìý
|
50.8
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
14.5
|
Ìý
|
Ìý
|
14.6
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Deferred income taxes |
1.2
|
Ìý
|
Ìý
|
1.2
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
121.3
|
Ìý
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
Other non-current assets |
1.3
|
Ìý
|
Ìý
|
1.5
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
138.3
|
Ìý
|
Ìý
|
138.6
|
Ìý
|
Ìý
|
TOTAL ASSETS |
200.5
|
Ìý
|
Ìý
|
205.3
|
Ìý
|
Ìý
|
Accounts payable |
4.4
|
Ìý
|
Ìý
|
4.2
|
Ìý
|
Ìý
|
Accrued employment costs |
0.1
|
Ìý
|
Ìý
|
0.1
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative liabilities |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
7.3
|
Ìý
|
Ìý
|
8.2
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
11.8
|
Ìý
|
Ìý
|
12.5
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
(244.3)
|
Ìý
|
Ìý
|
(230.0)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
19.4
|
Ìý
|
Ìý
|
19.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
6.1
|
Ìý
|
Ìý
|
8.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(207.0)
|
Ìý
|
Ìý
|
(188.8)
|
Ìý
|
Ìý
|
TOTAL EQUITY |
407.5
|
Ìý
|
Ìý
|
394.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
200.5
|
Ìý
|
Ìý
|
205.3
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.4
|
Ìý
|
Ìý
|
0.7
|
1.9
|
2.1
|
Accounts receivable, net |
161.4
|
Ìý
|
Ìý
|
221.3
|
Ìý
|
Ìý
|
Finished goods inventories |
162.2
|
Ìý
|
Ìý
|
77.8
|
Ìý
|
Ìý
|
Work-in-process inventories |
55.2
|
Ìý
|
Ìý
|
10.1
|
Ìý
|
Ìý
|
Supplies and other inventories |
110.8
|
Ìý
|
Ìý
|
93.2
|
Ìý
|
Ìý
|
Derivative assets |
72.8
|
Ìý
|
Ìý
|
91.4
|
Ìý
|
Ìý
|
Income tax receivable, current |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
22.9
|
Ìý
|
Ìý
|
16.9
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
585.7
|
Ìý
|
Ìý
|
511.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,706.8
|
Ìý
|
Ìý
|
1,221.9
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
27.1
|
Ìý
|
Ìý
|
68.4
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
4.1
|
Ìý
|
Ìý
|
4.1
|
Ìý
|
Ìý
|
Deferred income taxes |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
39.7
|
Ìý
|
Ìý
|
50.8
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
98.0
|
Ìý
|
Ìý
|
85.4
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
168.9
|
Ìý
|
Ìý
|
208.7
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,461.4
|
Ìý
|
Ìý
|
1,942.0
|
Ìý
|
Ìý
|
Accounts payable |
207.8
|
Ìý
|
Ìý
|
181.4
|
Ìý
|
Ìý
|
Accrued employment costs |
40.1
|
Ìý
|
Ìý
|
45.4
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative liabilities |
30.2
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
Ìý
|
43.5
|
Ìý
|
Ìý
|
Other current liabilities |
75.9
|
Ìý
|
Ìý
|
86.7
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
354.0
|
Ìý
|
Ìý
|
357.0
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
414.2
|
Ìý
|
Ìý
|
414.4
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
159.7
|
Ìý
|
Ìý
|
152.1
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
120.8
|
Ìý
|
Ìý
|
99.5
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
1,048.7
|
Ìý
|
Ìý
|
1,023.0
|
Ìý
|
Ìý
|
TOTAL EQUITY |
1,412.7
|
Ìý
|
Ìý
|
919.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
2,461.4
|
Ìý
|
Ìý
|
1,942.0
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
396.2
|
Ìý
|
Ìý
|
819.8
|
$ 892.5
|
$ 948.9
|
Accounts receivable, net |
7.4
|
Ìý
|
Ìý
|
9.2
|
Ìý
|
Ìý
|
Finished goods inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Work-in-process inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
Ìý
|
0.1
|
Ìý
|
Ìý
|
Income tax receivable, current |
58.7
|
Ìý
|
Ìý
|
117.3
|
Ìý
|
Ìý
|
Other current assets |
9.3
|
Ìý
|
Ìý
|
10.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
471.6
|
Ìý
|
Ìý
|
956.4
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
12.3
|
Ìý
|
Ìý
|
13.3
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable, non-current |
58.6
|
Ìý
|
Ìý
|
117.2
|
Ìý
|
Ìý
|
Deferred income taxes |
436.3
|
Ìý
|
Ìý
|
463.6
|
Ìý
|
Ìý
|
Investment in subsidiaries |
1,780.5
|
Ìý
|
Ìý
|
1,262.3
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
15.6
|
Ìý
|
Ìý
|
8.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
2,291.0
|
Ìý
|
Ìý
|
1,851.1
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,774.9
|
Ìý
|
Ìý
|
2,820.8
|
Ìý
|
Ìý
|
Accounts payable |
4.7
|
Ìý
|
Ìý
|
5.3
|
Ìý
|
Ìý
|
Accrued employment costs |
17.1
|
Ìý
|
Ìý
|
28.5
|
Ìý
|
Ìý
|
Accrued interest |
34.1
|
Ìý
|
Ìý
|
38.4
|
Ìý
|
Ìý
|
Derivative liabilities |
2.4
|
Ìý
|
Ìý
|
3.7
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
38.5
|
Ìý
|
Ìý
|
26.9
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
96.8
|
Ìý
|
Ìý
|
102.8
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
63.3
|
Ìý
|
Ìý
|
64.3
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,109.1
|
Ìý
|
Ìý
|
2,092.9
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
121.3
|
Ìý
|
Ìý
|
121.3
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
24.5
|
Ìý
|
Ìý
|
15.3
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
2,415.0
|
Ìý
|
Ìý
|
2,396.6
|
Ìý
|
Ìý
|
TOTAL EQUITY |
359.9
|
Ìý
|
Ìý
|
424.2
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND EQUITY |
$ 2,774.9
|
Ìý
|
Ìý
|
$ 2,820.8
|
Ìý
|
Ìý
|