SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Mar. 31, 2018 |
Jan. 01, 2018 |
Dec. 31, 2017 |
Mar. 31, 2017 |
Dec. 31, 2016 |
Cash and cash equivalents |
$ 786.6
|
$ 1,007.7
|
$ 1,007.7
|
$ 295.3
|
Ìý
|
Accounts receivable, net |
47.2
|
217.2
|
140.6
|
Ìý
|
Ìý
|
Inventories |
324.4
|
132.0
|
183.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
81.7
|
93.9
|
93.9
|
Ìý
|
Ìý
|
Derivative assets |
93.6
|
51.0
|
39.4
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
50.4
|
51.6
|
51.6
|
Ìý
|
Ìý
|
Other current assets |
28.5
|
28.0
|
28.0
|
Ìý
|
Ìý
|
Assets, Current |
1,412.4
|
1,581.4
|
1,544.6
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,047.3
|
1,051.0
|
1,051.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
74.1
|
17.8
|
17.8
|
Ìý
|
Ìý
|
Income Taxes Receivable |
219.9
|
Ìý
|
235.3
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
109.2
|
104.7
|
104.7
|
Ìý
|
Ìý
|
Total Other Assets |
403.2
|
357.8
|
357.8
|
Ìý
|
Ìý
|
Assets |
2,862.9
|
2,990.2
|
2,953.4
|
Ìý
|
Ìý
|
Accounts payable |
99.5
|
129.1
|
127.7
|
Ìý
|
Ìý
|
Accrued expenses |
94.4
|
107.1
|
107.1
|
Ìý
|
Ìý
|
Accrued interest |
28.2
|
31.4
|
31.4
|
Ìý
|
Ìý
|
Contingent claims |
54.3
|
55.6
|
55.6
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
44.2
|
44.2
|
Ìý
|
Ìý
|
Other current liabilities |
104.3
|
87.6
|
86.2
|
Ìý
|
Ìý
|
Liabilities, Current |
424.9
|
455.0
|
452.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
251.4
|
257.7
|
257.7
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
181.2
|
196.5
|
196.5
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,308.2
|
2,304.2
|
2,304.2
|
Ìý
|
Ìý
|
Long term intercompany notes payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
182.0
|
186.9
|
186.9
|
Ìý
|
Ìý
|
Liabilities |
3,347.7
|
3,400.3
|
3,397.5
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
(485.0)
|
(410.3)
|
(444.3)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Noncontrolling Interest |
0.2
|
0.2
|
0.2
|
Ìý
|
Ìý
|
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest |
(484.8)
|
(410.1)
|
(444.1)
|
$ (703.0)
|
$ (1,330.5)
|
Liabilities and Equity |
2,862.9
|
$ 2,990.2
|
2,953.4
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Accounts receivable, net |
(2.1)
|
Ìý
|
(0.7)
|
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
|
Loans to and accounts receivable from the Canadian Entities |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Assets, Current |
(2.1)
|
Ìý
|
(0.7)
|
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 Taxes Receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
(1,213.1)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
Long term intercompany notes |
(242.0)
|
Ìý
|
(242.0)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Total Other Assets |
(1,455.1)
|
Ìý
|
(1,296.2)
|
Ìý
|
Ìý
|
Assets |
(1,457.2)
|
Ìý
|
(1,296.9)
|
Ìý
|
Ìý
|
Accounts payable |
(2.1)
|
Ìý
|
(0.7)
|
Ìý
|
Ìý
|
Accrued expenses |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
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
|
Liabilities, Current |
(2.1)
|
Ìý
|
(0.7)
|
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 payable |
(242.0)
|
Ìý
|
(242.0)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Liabilities |
(244.1)
|
Ìý
|
(242.7)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
(1,213.1)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Noncontrolling Interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest |
(1,213.1)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
Liabilities and Equity |
(1,457.2)
|
Ìý
|
(1,296.9)
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
32.0
|
Ìý
|
56.7
|
Ìý
|
Ìý
|
Accounts receivable, net |
15.9
|
Ìý
|
33.9
|
Ìý
|
Ìý
|
Inventories |
21.2
|
Ìý
|
45.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
0.3
|
Ìý
|
5.1
|
Ìý
|
Ìý
|
Derivative assets |
0.0
|
Ìý
|
1.5
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
5.1
|
Ìý
|
4.1
|
Ìý
|
Ìý
|
Assets, Current |
74.5
|
Ìý
|
146.3
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
74.2
|
Ìý
|
74.5
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
72.2
|
Ìý
|
16.5
|
Ìý
|
Ìý
|
Income Taxes Receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long term intercompany notes |
242.0
|
Ìý
|
242.0
|
Ìý
|
Ìý
|
Other non-current assets |
2.4
|
Ìý
|
5.2
|
Ìý
|
Ìý
|
Total Other Assets |
316.6
|
Ìý
|
263.7
|
Ìý
|
Ìý
|
Assets |
465.3
|
Ìý
|
484.5
|
Ìý
|
Ìý
|
Accounts payable |
25.2
|
Ìý
|
31.6
|
Ìý
|
Ìý
|
Accrued expenses |
23.3
|
Ìý
|
28.2
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
39.2
|
Ìý
|
20.6
|
Ìý
|
Ìý
|
Liabilities, Current |
87.7
|
Ìý
|
80.4
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
(244.3)
|
Ìý
|
(239.3)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
38.1
|
Ìý
|
55.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long term intercompany notes payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
21.6
|
Ìý
|
24.0
|
Ìý
|
Ìý
|
Liabilities |
(96.9)
|
Ìý
|
(79.0)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
562.0
|
Ìý
|
563.3
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Noncontrolling Interest |
0.2
|
Ìý
|
0.2
|
Ìý
|
Ìý
|
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest |
562.2
|
Ìý
|
563.5
|
Ìý
|
Ìý
|
Liabilities and Equity |
465.3
|
Ìý
|
484.5
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
1.0
|
Ìý
|
2.1
|
Ìý
|
Ìý
|
Accounts receivable, net |
27.7
|
Ìý
|
102.9
|
Ìý
|
Ìý
|
Inventories |
303.2
|
Ìý
|
138.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
81.4
|
Ìý
|
88.8
|
Ìý
|
Ìý
|
Derivative assets |
93.6
|
Ìý
|
37.9
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
6.9
|
Ìý
|
6.9
|
Ìý
|
Ìý
|
Other current assets |
7.9
|
Ìý
|
7.5
|
Ìý
|
Ìý
|
Assets, Current |
521.7
|
Ìý
|
384.5
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
956.9
|
Ìý
|
959.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
1.9
|
Ìý
|
1.3
|
Ìý
|
Ìý
|
Income Taxes Receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
27.4
|
Ìý
|
29.9
|
Ìý
|
Ìý
|
Long term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
97.9
|
Ìý
|
91.7
|
Ìý
|
Ìý
|
Total Other Assets |
127.2
|
Ìý
|
122.9
|
Ìý
|
Ìý
|
Assets |
1,605.8
|
Ìý
|
1,466.4
|
Ìý
|
Ìý
|
Accounts payable |
71.8
|
Ìý
|
89.7
|
Ìý
|
Ìý
|
Accrued expenses |
60.0
|
Ìý
|
59.9
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
Ìý
|
44.2
|
Ìý
|
Ìý
|
Other current liabilities |
63.3
|
Ìý
|
63.5
|
Ìý
|
Ìý
|
Liabilities, Current |
239.3
|
Ìý
|
257.3
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
429.6
|
Ìý
|
430.6
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
143.1
|
Ìý
|
140.6
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long term intercompany notes payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
142.9
|
Ìý
|
147.2
|
Ìý
|
Ìý
|
Liabilities |
954.9
|
Ìý
|
975.7
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
650.9
|
Ìý
|
490.7
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Noncontrolling Interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest |
650.9
|
Ìý
|
490.7
|
Ìý
|
Ìý
|
Liabilities and Equity |
1,605.8
|
Ìý
|
1,466.4
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
753.6
|
Ìý
|
948.9
|
Ìý
|
Ìý
|
Accounts receivable, net |
5.7
|
Ìý
|
4.5
|
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
|
Loans to and accounts receivable from the Canadian Entities |
43.5
|
Ìý
|
44.7
|
Ìý
|
Ìý
|
Other current assets |
15.5
|
Ìý
|
16.4
|
Ìý
|
Ìý
|
Assets, Current |
818.3
|
Ìý
|
1,014.5
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
16.2
|
Ìý
|
17.5
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income Taxes Receivable |
219.9
|
Ìý
|
235.3
|
Ìý
|
Ìý
|
Investment in subsidiaries |
1,185.7
|
Ìý
|
1,024.3
|
Ìý
|
Ìý
|
Long term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
8.9
|
Ìý
|
7.8
|
Ìý
|
Ìý
|
Total Other Assets |
1,414.5
|
Ìý
|
1,267.4
|
Ìý
|
Ìý
|
Assets |
2,249.0
|
Ìý
|
2,299.4
|
Ìý
|
Ìý
|
Accounts payable |
4.6
|
Ìý
|
7.1
|
Ìý
|
Ìý
|
Accrued expenses |
11.1
|
Ìý
|
19.0
|
Ìý
|
Ìý
|
Accrued interest |
28.2
|
Ìý
|
31.4
|
Ìý
|
Ìý
|
Contingent claims |
54.3
|
Ìý
|
55.6
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
1.8
|
Ìý
|
2.1
|
Ìý
|
Ìý
|
Liabilities, Current |
100.0
|
Ìý
|
115.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
66.1
|
Ìý
|
66.4
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,308.2
|
Ìý
|
2,304.2
|
Ìý
|
Ìý
|
Long term intercompany notes payable |
242.0
|
Ìý
|
242.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
17.5
|
Ìý
|
15.7
|
Ìý
|
Ìý
|
Liabilities |
2,733.8
|
Ìý
|
2,743.5
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Parent |
(484.8)
|
Ìý
|
(444.1)
|
Ìý
|
Ìý
|
Stockholders' Equity Attributable to Noncontrolling Interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest |
(484.8)
|
Ìý
|
(444.1)
|
Ìý
|
Ìý
|
Liabilities and Equity |
$ 2,249.0
|
Ìý
|
$ 2,299.4
|
Ìý
|
Ìý
|