SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($) $ in Millions |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
CURRENT ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
$ 1,007.7
|
$ 323.4
|
Ìý
|
Ìý
|
Accounts receivable, net |
140.6
|
128.7
|
Ìý
|
Ìý
|
Inventories |
183.4
|
178.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
93.9
|
91.4
|
Ìý
|
Ìý
|
Derivative assets |
39.4
|
33.1
|
Ìý
|
Ìý
|
Loans to and accounts receivables from the Canadian Entities |
51.6
|
48.6
|
Ìý
|
Ìý
|
Other current assets |
28.0
|
21.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,544.6
|
824.6
|
Ìý
|
Ìý
|
OTHER ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
1,051.0
|
984.4
|
$ 1,059.0
|
Ìý
|
INCOME TAX RECEIVABLE |
235.3
|
0.0
|
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 |
122.5
|
114.9
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,953.4
|
1,923.9
|
2,135.5
|
Ìý
|
CURRENT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
127.7
|
107.6
|
Ìý
|
Ìý
|
Accrued employment costs |
56.1
|
56.1
|
Ìý
|
Ìý
|
State and local taxes payable |
30.2
|
28.3
|
Ìý
|
Ìý
|
Accrued expenses |
33.7
|
41.1
|
Ìý
|
Ìý
|
Accrued interest |
31.4
|
40.2
|
Ìý
|
Ìý
|
Accrued royalties |
17.3
|
26.2
|
Ìý
|
Ìý
|
Contingent claims |
55.6
|
0.0
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
8.7
|
Ìý
|
Ìý
|
Other current liabilities |
56.0
|
82.9
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
452.2
|
391.1
|
Ìý
|
Ìý
|
POSTEMPLOYMENT BENEFIT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Pensions |
222.8
|
245.7
|
Ìý
|
Ìý
|
Other postretirement benefits |
34.9
|
34.8
|
Ìý
|
Ìý
|
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |
257.7
|
280.5
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
196.5
|
193.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,304.2
|
2,175.1
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
186.9
|
213.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
3,397.5
|
3,254.4
|
Ìý
|
Ìý
|
CLIFFS SHAREHOLDERS' DEFICIT |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(444.3)
|
(1,464.3)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.2
|
133.8
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(444.1)
|
(1,330.5)
|
$ (1,811.6)
|
$ (1,734.3)
|
TOTAL LIABILITIES AND DEFICIT |
2,953.4
|
1,923.9
|
Ìý
|
Ìý
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
CURRENT ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accounts receivable, net |
(0.7)
|
(0.8)
|
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
|
Loans to and accounts receivables from the Canadian Entities |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
(0.7)
|
(0.8)
|
Ìý
|
Ìý
|
OTHER ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
0.0
|
0.0
|
Ìý
|
Ìý
|
INCOME TAX RECEIVABLE |
0.0
|
Ìý
|
Ìý
|
Ìý
|
INVESTMENT IN SUBSIDIARIES |
(1,054.2)
|
(907.0)
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
(242.0)
|
(197.0)
|
Ìý
|
Ìý
|
Other non-current assets |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
(1,296.9)
|
(1,104.8)
|
Ìý
|
Ìý
|
CURRENT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
(0.7)
|
(0.8)
|
Ìý
|
Ìý
|
Accrued employment costs |
0.0
|
0.0
|
Ìý
|
Ìý
|
State and local taxes payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued expenses |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued royalties |
0.0
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
Ìý
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
(0.7)
|
(0.8)
|
Ìý
|
Ìý
|
POSTEMPLOYMENT BENEFIT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Pensions |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other postretirement benefits |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL 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 |
(242.0)
|
(197.0)
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(242.7)
|
(197.8)
|
Ìý
|
Ìý
|
CLIFFS SHAREHOLDERS' DEFICIT |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(1,054.2)
|
(907.0)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(1,054.2)
|
(907.0)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
(1,296.9)
|
(1,104.8)
|
Ìý
|
Ìý
|
Parent [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
CURRENT ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
948.9
|
283.4
|
Ìý
|
Ìý
|
Accounts receivable, net |
4.5
|
4.5
|
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
|
Loans to and accounts receivables from the Canadian Entities |
44.7
|
41.7
|
Ìý
|
Ìý
|
Other current assets |
16.4
|
8.6
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
1,014.5
|
338.2
|
Ìý
|
Ìý
|
OTHER ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
17.5
|
21.4
|
Ìý
|
Ìý
|
INCOME TAX RECEIVABLE |
235.3
|
Ìý
|
Ìý
|
Ìý
|
INVESTMENT IN SUBSIDIARIES |
1,024.3
|
882.4
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
7.8
|
11.0
|
Ìý
|
Ìý
|
TOTAL ASSETS |
2,299.4
|
1,253.0
|
Ìý
|
Ìý
|
CURRENT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
7.1
|
1.6
|
Ìý
|
Ìý
|
Accrued employment costs |
13.7
|
15.6
|
Ìý
|
Ìý
|
State and local taxes payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued expenses |
5.3
|
7.6
|
Ìý
|
Ìý
|
Accrued interest |
31.4
|
40.2
|
Ìý
|
Ìý
|
Accrued royalties |
0.0
|
0.0
|
Ìý
|
Ìý
|
Contingent claims |
55.6
|
Ìý
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
2.1
|
23.0
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
115.2
|
88.0
|
Ìý
|
Ìý
|
POSTEMPLOYMENT BENEFIT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Pensions |
59.2
|
56.9
|
Ìý
|
Ìý
|
Other postretirement benefits |
7.2
|
7.6
|
Ìý
|
Ìý
|
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |
66.4
|
64.5
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
2,304.2
|
2,175.1
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
242.0
|
197.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
15.7
|
58.9
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
2,743.5
|
2,583.5
|
Ìý
|
Ìý
|
CLIFFS SHAREHOLDERS' DEFICIT |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
(444.1)
|
(1,330.5)
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.0
|
0.0
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
(444.1)
|
(1,330.5)
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
2,299.4
|
1,253.0
|
Ìý
|
Ìý
|
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
CURRENT ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
2.1
|
2.5
|
Ìý
|
Ìý
|
Accounts receivable, net |
102.9
|
59.3
|
Ìý
|
Ìý
|
Inventories |
138.4
|
137.0
|
Ìý
|
Ìý
|
Supplies and other inventories |
88.8
|
86.4
|
Ìý
|
Ìý
|
Derivative assets |
37.9
|
31.7
|
Ìý
|
Ìý
|
Loans to and accounts receivables from the Canadian Entities |
6.9
|
6.9
|
Ìý
|
Ìý
|
Other current assets |
7.5
|
8.2
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
384.5
|
332.0
|
Ìý
|
Ìý
|
OTHER ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
959.0
|
937.7
|
Ìý
|
Ìý
|
INCOME TAX RECEIVABLE |
0.0
|
Ìý
|
Ìý
|
Ìý
|
INVESTMENT IN SUBSIDIARIES |
29.9
|
24.6
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other non-current assets |
93.0
|
94.1
|
Ìý
|
Ìý
|
TOTAL ASSETS |
1,466.4
|
1,388.4
|
Ìý
|
Ìý
|
CURRENT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
89.7
|
92.6
|
Ìý
|
Ìý
|
Accrued employment costs |
38.9
|
34.6
|
Ìý
|
Ìý
|
State and local taxes payable |
30.0
|
28.1
|
Ìý
|
Ìý
|
Accrued expenses |
13.2
|
14.4
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued royalties |
7.8
|
13.0
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
Ìý
|
Ìý
|
Ìý
|
Partnership distribution payable |
44.2
|
8.7
|
Ìý
|
Ìý
|
Other current liabilities |
33.5
|
35.3
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
257.3
|
226.7
|
Ìý
|
Ìý
|
POSTEMPLOYMENT BENEFIT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Pensions |
403.6
|
397.4
|
Ìý
|
Ìý
|
Other postretirement benefits |
27.0
|
26.5
|
Ìý
|
Ìý
|
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |
430.6
|
423.9
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
140.6
|
153.9
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
147.2
|
118.8
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
975.7
|
923.3
|
Ìý
|
Ìý
|
CLIFFS SHAREHOLDERS' DEFICIT |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
490.7
|
331.5
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.0
|
133.6
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
490.7
|
465.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
1,466.4
|
1,388.4
|
Ìý
|
Ìý
|
Non-Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
CURRENT ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Cash and cash equivalents |
56.7
|
37.5
|
Ìý
|
Ìý
|
Accounts receivable, net |
33.9
|
65.7
|
Ìý
|
Ìý
|
Inventories |
45.0
|
41.4
|
Ìý
|
Ìý
|
Supplies and other inventories |
5.1
|
5.0
|
Ìý
|
Ìý
|
Derivative assets |
1.5
|
1.4
|
Ìý
|
Ìý
|
Loans to and accounts receivables from the Canadian Entities |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current assets |
4.1
|
4.2
|
Ìý
|
Ìý
|
TOTAL CURRENT ASSETS |
146.3
|
155.2
|
Ìý
|
Ìý
|
OTHER ASSETS |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
PROPERTY, PLANT AND EQUIPMENT, NET |
74.5
|
25.3
|
Ìý
|
Ìý
|
INCOME TAX RECEIVABLE |
0.0
|
Ìý
|
Ìý
|
Ìý
|
INVESTMENT IN SUBSIDIARIES |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
242.0
|
197.0
|
Ìý
|
Ìý
|
Other non-current assets |
21.7
|
9.8
|
Ìý
|
Ìý
|
TOTAL ASSETS |
484.5
|
387.3
|
Ìý
|
Ìý
|
CURRENT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Accounts payable |
31.6
|
14.2
|
Ìý
|
Ìý
|
Accrued employment costs |
3.5
|
5.9
|
Ìý
|
Ìý
|
State and local taxes payable |
0.2
|
0.2
|
Ìý
|
Ìý
|
Accrued expenses |
15.2
|
19.1
|
Ìý
|
Ìý
|
Accrued interest |
0.0
|
0.0
|
Ìý
|
Ìý
|
Accrued royalties |
9.5
|
13.2
|
Ìý
|
Ìý
|
Contingent claims |
0.0
|
Ìý
|
Ìý
|
Ìý
|
Partnership distribution payable |
0.0
|
0.0
|
Ìý
|
Ìý
|
Other current liabilities |
20.4
|
24.6
|
Ìý
|
Ìý
|
TOTAL CURRENT LIABILITIES |
80.4
|
77.2
|
Ìý
|
Ìý
|
POSTEMPLOYMENT BENEFIT LIABILITIES |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Pensions |
(240.0)
|
(208.6)
|
Ìý
|
Ìý
|
Other postretirement benefits |
0.7
|
0.7
|
Ìý
|
Ìý
|
TOTAL POSTEMPLOYMENT BENEFIT LIABILITIES |
(239.3)
|
(207.9)
|
Ìý
|
Ìý
|
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS |
55.9
|
40.0
|
Ìý
|
Ìý
|
LONG-TERM DEBT |
0.0
|
0.0
|
Ìý
|
Ìý
|
LONG-TERM INTERCOMPANY NOTES |
0.0
|
0.0
|
Ìý
|
Ìý
|
OTHER LIABILITIES |
24.0
|
36.1
|
Ìý
|
Ìý
|
TOTAL LIABILITIES |
(79.0)
|
(54.6)
|
Ìý
|
Ìý
|
CLIFFS SHAREHOLDERS' DEFICIT |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
TOTAL CLIFFS SHAREHOLDERS' DEFICIT |
563.3
|
441.7
|
Ìý
|
Ìý
|
NONCONTROLLING INTEREST |
0.2
|
0.2
|
Ìý
|
Ìý
|
TOTAL DEFICIT |
563.5
|
441.9
|
Ìý
|
Ìý
|
TOTAL LIABILITIES AND DEFICIT |
$ 484.5
|
$ 387.3
|
Ìý
|
Ìý
|