SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Jun. 30, 2018 |
Jan. 01, 2018 |
Dec. 31, 2017 |
Jun. 30, 2017 |
Dec. 31, 2016 |
Cash and cash equivalents |
$ 802.5
|
$ 978.3
|
$ 978.3
|
$ 270.4
|
Ìý
|
Accounts receivable, net |
152.6
|
183.3
|
106.7
|
Ìý
|
Ìý
|
Inventories |
256.4
|
87.0
|
138.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
88.6
|
88.8
|
88.8
|
Ìý
|
Ìý
|
Derivative assets |
174.7
|
49.5
|
37.9
|
Ìý
|
Ìý
|
Current assets of discontinued operations |
45.3
|
118.5
|
118.5
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
0.0
|
51.6
|
51.6
|
Ìý
|
Ìý
|
Other current assets |
26.8
|
24.4
|
24.4
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,546.9
|
1,581.4
|
1,544.6
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,081.3
|
1,033.8
|
1,033.8
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
85.7
|
17.8
|
17.8
|
Ìý
|
Ìý
|
Income tax receivable |
219.9
|
Ìý
|
235.3
|
Ìý
|
Ìý
|
Non-current assets of discontinued operations |
0.0
|
20.3
|
20.3
|
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 |
117.7
|
101.6
|
101.6
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
423.3
|
375.0
|
375.0
|
Ìý
|
Ìý
|
Assets |
3,051.5
|
2,990.2
|
2,953.4
|
Ìý
|
Ìý
|
Accounts payable |
119.0
|
100.9
|
99.5
|
Ìý
|
Ìý
|
Accrued expenses |
85.1
|
79.1
|
79.1
|
Ìý
|
Ìý
|
Accrued interest |
43.1
|
31.4
|
31.4
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
55.6
|
55.6
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
44.2
|
44.2
|
Ìý
|
Ìý
|
Current liabilities of discontinued operations |
117.3
|
75.0
|
75.0
|
Ìý
|
Ìý
|
Other current liabilities |
66.2
|
68.8
|
67.4
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
474.9
|
455.0
|
452.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
245.0
|
257.7
|
257.7
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
172.3
|
167.7
|
167.7
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,297.0
|
2,304.2
|
2,304.2
|
Ìý
|
Ìý
|
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS |
10.3
|
52.2
|
52.2
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
158.3
|
163.5
|
163.5
|
Ìý
|
Ìý
|
Liabilities |
3,357.8
|
3,400.3
|
3,397.5
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(306.3)
|
(410.3)
|
(444.3)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.0
|
0.2
|
0.2
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(306.3)
|
(410.1)
|
(444.1)
|
$ (666.7)
|
$ (1,330.5)
|
TOTAL LIABILITIES AND DEFICIT |
3,051.5
|
$ 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 |
(3.7)
|
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
|
Current assets of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
(3.7)
|
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 tax receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Non-current assets of discontinued operations |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
(1,332.7)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
Long-term intercompany notes |
(227.4)
|
Ìý
|
(242.0)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
(1,560.1)
|
Ìý
|
(1,296.2)
|
Ìý
|
Ìý
|
Assets |
(1,563.8)
|
Ìý
|
(1,296.9)
|
Ìý
|
Ìý
|
Accounts payable |
(3.7)
|
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 |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Current liabilities of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
(3.7)
|
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
|
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(227.4)
|
Ìý
|
(242.0)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Liabilities |
(231.1)
|
Ìý
|
(242.7)
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(1,332.7)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(1,332.7)
|
Ìý
|
(1,054.2)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
(1,563.8)
|
Ìý
|
(1,296.9)
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.5
|
Ìý
|
27.3
|
Ìý
|
Ìý
|
Accounts receivable, net |
0.1
|
Ìý
|
0.0
|
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
|
Current assets of discontinued operations |
45.3
|
Ìý
|
118.5
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
3.2
|
Ìý
|
0.5
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
51.1
|
Ìý
|
146.3
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
99.6
|
Ìý
|
57.3
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
81.9
|
Ìý
|
16.5
|
Ìý
|
Ìý
|
Income tax receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Non-current assets of discontinued operations |
Ìý
|
Ìý
|
20.3
|
Ìý
|
Ìý
|
Investment in subsidiaries |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Long-term intercompany notes |
227.4
|
Ìý
|
242.0
|
Ìý
|
Ìý
|
Other non-current assets |
1.7
|
Ìý
|
2.1
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
311.0
|
Ìý
|
280.9
|
Ìý
|
Ìý
|
Assets |
461.7
|
Ìý
|
484.5
|
Ìý
|
Ìý
|
Accounts payable |
28.7
|
Ìý
|
3.4
|
Ìý
|
Ìý
|
Accrued expenses |
0.5
|
Ìý
|
0.2
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Current liabilities of discontinued operations |
117.3
|
Ìý
|
75.0
|
Ìý
|
Ìý
|
Other current liabilities |
1.7
|
Ìý
|
1.8
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
148.2
|
Ìý
|
80.4
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
(249.3)
|
Ìý
|
(239.3)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
27.0
|
Ìý
|
27.1
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS |
10.3
|
Ìý
|
52.2
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.6
|
Ìý
|
0.6
|
Ìý
|
Ìý
|
Liabilities |
(63.2)
|
Ìý
|
(79.0)
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
524.9
|
Ìý
|
563.3
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
Ìý
|
Ìý
|
0.2
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
524.9
|
Ìý
|
563.5
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
461.7
|
Ìý
|
484.5
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
1.6
|
Ìý
|
2.1
|
Ìý
|
Ìý
|
Accounts receivable, net |
148.2
|
Ìý
|
102.9
|
Ìý
|
Ìý
|
Inventories |
256.4
|
Ìý
|
138.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
88.6
|
Ìý
|
88.8
|
Ìý
|
Ìý
|
Derivative assets |
174.6
|
Ìý
|
37.9
|
Ìý
|
Ìý
|
Current assets of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
Ìý
|
Ìý
|
6.9
|
Ìý
|
Ìý
|
Other current assets |
7.7
|
Ìý
|
7.5
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
677.1
|
Ìý
|
384.5
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
966.2
|
Ìý
|
959.0
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
3.8
|
Ìý
|
1.3
|
Ìý
|
Ìý
|
Income tax receivable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Non-current assets of discontinued operations |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
32.8
|
Ìý
|
29.9
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
107.3
|
Ìý
|
91.7
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
143.9
|
Ìý
|
122.9
|
Ìý
|
Ìý
|
Assets |
1,787.2
|
Ìý
|
1,466.4
|
Ìý
|
Ìý
|
Accounts payable |
88.8
|
Ìý
|
89.7
|
Ìý
|
Ìý
|
Accrued expenses |
71.0
|
Ìý
|
59.9
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
Ìý
|
44.2
|
Ìý
|
Ìý
|
Current liabilities of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
62.7
|
Ìý
|
63.5
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
266.7
|
Ìý
|
257.3
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
428.4
|
Ìý
|
430.6
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
145.3
|
Ìý
|
140.6
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
139.0
|
Ìý
|
147.2
|
Ìý
|
Ìý
|
Liabilities |
979.4
|
Ìý
|
975.7
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
807.8
|
Ìý
|
490.7
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
807.8
|
Ìý
|
490.7
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
1,787.2
|
Ìý
|
1,466.4
|
Ìý
|
Ìý
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
798.4
|
Ìý
|
948.9
|
Ìý
|
Ìý
|
Accounts receivable, net |
8.0
|
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 |
0.1
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Current assets of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Loans to and accounts receivable from the Canadian Entities |
Ìý
|
Ìý
|
44.7
|
Ìý
|
Ìý
|
Other current assets |
15.9
|
Ìý
|
16.4
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
822.4
|
Ìý
|
1,014.5
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
15.5
|
Ìý
|
17.5
|
Ìý
|
Ìý
|
Deposits for property, plant and equipment |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Income tax receivable |
219.9
|
Ìý
|
235.3
|
Ìý
|
Ìý
|
Non-current assets of discontinued operations |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Investment in subsidiaries |
1,299.9
|
Ìý
|
1,024.3
|
Ìý
|
Ìý
|
Long-term intercompany notes |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
8.7
|
Ìý
|
7.8
|
Ìý
|
Ìý
|
TOTAL OTHER ASSETS |
1,528.5
|
Ìý
|
1,267.4
|
Ìý
|
Ìý
|
Assets |
2,366.4
|
Ìý
|
2,299.4
|
Ìý
|
Ìý
|
Accounts payable |
5.2
|
Ìý
|
7.1
|
Ìý
|
Ìý
|
Accrued expenses |
13.6
|
Ìý
|
19.0
|
Ìý
|
Ìý
|
Accrued interest |
43.1
|
Ìý
|
31.4
|
Ìý
|
Ìý
|
Contingent claims |
Ìý
|
Ìý
|
55.6
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Current liabilities of discontinued operations |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
1.8
|
Ìý
|
2.1
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
63.7
|
Ìý
|
115.2
|
Ìý
|
Ìý
|
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES |
65.9
|
Ìý
|
66.4
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,297.0
|
Ìý
|
2,304.2
|
Ìý
|
Ìý
|
NON-CURRENT LIABILITIES OF DISCONTINUED OPERATIONS |
0.0
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
227.4
|
Ìý
|
242.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
18.7
|
Ìý
|
15.7
|
Ìý
|
Ìý
|
Liabilities |
2,672.7
|
Ìý
|
2,743.5
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(306.3)
|
Ìý
|
(444.1)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
Ìý
|
Ìý
|
0.0
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(306.3)
|
Ìý
|
(444.1)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
$ 2,366.4
|
Ìý
|
$ 2,299.4
|
Ìý
|
Ìý
|